The Brainteaser Elon Musk Asks New SpaceX Engineers

Nerval's Lobster writes: The latest biography of Elon Musk, by technology journalist Ashlee Vance, provides an in-depth look into how the entrepreneur and tech titan built Tesla Motors and SpaceX from the ground up. For developers and engineers, getting a job at SpaceX is difficult, with a long interviewing/testing process... and for some candidates, there's a rather unique final step: an interview with Musk himself. During that interview, Musk reportedly likes to ask candidates a particular brainteaser: "You're standing on the surface of the Earth. You walk one mile south, one mile west, and one mile north. You end up exactly where you started. Where are you?" If you can answer that riddle successfully, and pass all of SpaceX's other stringent tests, you may have a shot at launching rockets into orbit.

Comments

North Pole

[605dave]

I am guessing the answer is the north pole...

Re:North Pole

[Jamu]

or a treadmill, but you'd have to turn it 90 degrees clockwise twice.

Re:North Pole

[Penguinisto]

The answer is indeed the North Pole, and that brain teaser has been around for what, eons now?

I think I'd quickly answer it, then ask him one that I made up and tested long before that final interview.

Re:North Pole

[HornWumpus]

You all fail.

Re:North Pole

[Penguinisto]

Ice is a legitimate part of the a surface of the Earth and the math works out.

How is that failure?

Re:North Pole

[abelenky17]

You'd only be partially correct.

There are actually multiple solutions:

1.) North Pole (one mile south, one west, and one north brings you back to the north pole)
2.) A ring of points approximately 2 miles just north of the the south pole, such that when you walk one mile south, you're even closer to the pole, then walk one mile west, going completely "around the world", back to where you started your westward travel, and one mile north, bringing you back to your original position.

Re:North Pole

[605dave]

Then what's the answer?

Re:North Pole

[HornWumpus]

The north pole and a circle of lat 1 + 1 / (2 * PI) north of the south pole. The distance is an approximation but is 'close enough for rocket science'. When you walk east you circumnavigate.

Re:North Pole

[viperidaenz]

Not eons, Elons

Re:North Pole

[softarch]

Or another ring of points closer to the south pole such that you go completely around the world twice, and another ring closer such that you go around the world 3 times....

Re:North Pole

[Hussman32]

I think the more correct answer is the magnetic north pole using a compass for determining your direction. Nobody said the turns had to be 90 degrees.

Re:North Pole

[IcyWolfy]

That's the asymptote that needs to be addressed separate due to a division by 0.

The correct answer is an infinite number of points around the south pole, with the exception of the south pole itself, and the north pole.

Or in words:

Given C = 1 mile, there is an longitudinal (East-West) circle north around the south pole with a circumference of 1 mile.
Any point on this line is an answer.
As does any whole divisor of this (1/3 mile circumference is traversed three times in one mile, but back at the same starting point.)

So C(1/1) + C(1/2) + C(1/3) + C(1/4) + C(1/n)
And more generally
= C(1/n) where n != 0 is a circle around the south pole, and n==0 is the north pole solution, whose division by 0 needs resolution by analysis (which is more obvious)

Re: North Pole

[soniCron88]

It's certainly some point near the South Pole, since the North Pole hasn't got any earth. I'm not good enough with math to figure it out, but it's some point greater than one mile North of the South Pole such that when you do the one mile walk West, you cross all the longitudes and end up where you started. Then when you walk North again, you're back where you were in the beginning. Why is this more correct than starting at the North Pole? The same reason why Musk thought it better to rely on thrusters broom-balancing a landing rather than a simple parachute: he likes to do things the hard way!

Re:North Pole

[andymadigan]

It's definitely the north pole. If you start at the north pole and walk one mile south you'll be standing on some ice one mile from the pole, if you then walk west you'll actually be walking around a circle (look at lines of latitude, that is East-West lines on a globe near the poles). If you then walk 1 mile north you'll be right back at the north pole.

Took me longer to write the explanation than to figure it out, honestly a fifth grader could figure that out.

Re:North Pole

[Roger W Moore]

The north pole and a circle of lat 1 + 1 / (2 * PI) north of the south pole.

Actually the answer is the north pole and a circles of lat 1 + 1 / (2*pi*n) north of the south pole where n=1,2,3,4... etc. plus there is a slight correction because the surface of the earth is not entirely flat and so the circumference of a line of latitude is actually less than 2*pi*s where s is the arc length from the line to the south pole for the distances involved it would probably be negligible compared to surface defects.

Re:North Pole

[RatPh!nk]

An 'easier' answer, might bend the rules a little bit..... If you walked a mile to the south you would be 1 mi S. of equator. Then 1 mi W and you are still 1 mile S of equator. Then 1 mi N and you are again on the "exactly" on the equator - 0.

Just a thought.

Re:North Pole

[Anonymous Coward]

Nope, those rings are all more than a mile north of the south pole, so there is plenty of room to walk a mile south. The second ring has you walk south to a point close enough to the south pole that your mile west goes around twice, but you start a mile north of the point you can circumnavigate twice.

Re:North Pole

[BlueCoder]

The second theory for the question as a whole is invalid since the question implies a single spot rather than a ring of spots but I doubt he would deduct points for it.

Re:North Pole

[605dave]

Sweet, I'm as smart as a 5th grader!

Re:North Pole

[msauve]

The N pole is only the simple answer. Fact is, there are infinite points which meet the condition. Anywhere on the latitude 1+1/pi (add just a tiny bit more because of spherical geometry) miles north of the S pole also works. When you walk a mile W, you're making a full circle back to your original longitude. There are similar others, where you circle twice, etc.

Re:North Pole

[suutar]

That's one answer. Another is 1+(1/(2*pi*N)) miles north of the south pole (for integer N); you go one mile south, one mile west is N laps around the pole, and one mile north gets you back to your starting point.

Re:North Pole

[Austerity Empowers]

There are two north poles, and he specifically mentioned earth.

Re:North Pole

[codeAlDente]

But are these answers valid if the ring of points lies on a discontinuous surface, of if the surface is unwalkable?

Re:North Pole

[asliarun]

The answer is indeed the North Pole, and that brain teaser has been around for what, eons now?

I think I'd quickly answer it, then ask him one that I made up and tested long before that final interview.

And now it has been around for elons as well.

Re:North Pole

[firewrought]

Doing some quick calcs, the inner rings would all lie within 1/6th mile of the South Pole; the starting rings would lie between 1 and ~1.16 miles away. (1 mile is the degenerate case, where you have to spin in place an infinite amount to "walk" 1 mile, so this probably shouldn't count.)

Of course, any elevation change in the terrain you're walking across would cause these rings to distort and contract inward. (E.g., the rings you calculate for a perfect sphere are going to be larger than the equivalent rings you calculate if you're going to count "walking distance" up and down slopes.)

Re:North Pole

[xevioso]

Yes but not exactly where you started, which is a requirement. Although one could argue it's impossible to do so as the earth is moving through space (and time) so I would ask for a clarification from Mr. Musk before answering: "...exactly where I started, relative to WHAT, Mr. Musk?"

If he says simply, "well, relative to where you started - EXACTLY where you started!" I would sy it's impossible and say it's a trick question.

Re:North Pole

[codeAlDente]

You could probably even figure out the sum of all paths by using a geometric series.

Re:North Pole

[Austerity Empowers]

I think earth is an oblate spheroid, so it must be the geographic north pole, which would have the property of being "a triangle" and putting you back where you started. But what hurts my head is when we use terms like north/south/west we're referring to magnetic north (usually), and I'm not sure as you make the traversal he describes you actually end up back where you started. Also i'm not sure how to rule out the south pole (geographic) you could do this same exercise.

It can't be magnetic, the earth is somewhat and unequally distended around the magnetic poles such that you don't end up exactly at the starting point by taking the indicated path, you will end up every so slightly off.

Re:North Pole

[Duhavid]

"If he says simply, "well, relative to where you started - EXACTLY where you started!" I would sy it's impossible and say it's a trick question."

It's not, if he allows you to move infinity fast ( I.E., no time elapse for motion ).

Re:North Pole

[eggegick]

Very nice. I saw the first answer, "North Pole", as soon as I read the question, and did not think it much of a trick question for someone interviewing at a place like SpaceX. But these answers you guys offer are cool. So the question ought to be, "... other than the north pole, where might you be?" Still, I hate interview trick questions.

Re: North Pole

[Zero__Kelvin]

Close! The correct answer is I'm dead, because I was an idiot who tried to walk one mile south, one mile west, and one mile north at the North Pole.

Re:North Pole

[ChrisK87]

Nope. Those "inner rings" wouldn't allow you to walk south for one mile, so they are invalid.

This is false. The rings in question are north of the south pole, and you start out exactly one mile north of them, not on them. Your starting point is always one mile plus [some distance] from the pole, such that after the first one mile trek toward the pole, you are standing on the ring without having crossed the pole. See this mspaint illustration (which ignores scale with reckless abandon). The bullseye is the pole, and the straight segment is always walked first to get onto the 1/n-mile-circumference ring, which is walked n times, and then the straight segment is walked back north.

There are an infinite number of starting latitudes around the south pole corresponding to any integer value of n, and obviously every longitude is valid.

Re: North Pole

[Culture20]

Who are three people who have never been in my mother's kitchen?

Re:North Pole

[Tablizer]

Answer: "You end up in a black hole due to division by zero."

Re: North Pole

[Zero__Kelvin]

It's certainly some point near the South Pole, since the North Pole hasn't got any earth.

That would seem to be a problem, but is it as insurmountable as the fact that the South Pole doesn't have a South? (and anywhere near it would suggest many possible answers, none of which could be determined during an interview.) FTA:

You're standing on the surface of the Earth

He doesn't say what that surface is composed of, and I'm willing to bet that it is not that kind of trick question, since that would be a stupid way to asses someone's intellectual prowess to say the least.

Re:North Pole

[camperdave]

You are misunderstanding the starting point. The starting point is one mile north of these rings.

The other, less common answer, is that there are an infinite number of places on the Earth, where you would end up at the starting location if you were to travel one mile south, west, then north. And that is anywhere 1.159 miles north of the South Pole. You would travel south for one mile, putting you at .159 miles north of the South Pole. Then travelling one mile west would cause you to make a complete circle around the South Pole, ending where the westward mile started. Then travel one mile north and that would put you back at your original starting point.

Re:North Pole

[roman_mir]

Pffft. I would answer like so: I am standing on a cone located exactly on top of North pole. The cone is 1 mile tall.

Re:North Pole

[TWX]

You know, with that attitude they're probably not going to give you the job, Rincewind...

Re: North Pole

[Trogre]

Who said anything about needing earth beneath your feet? The North Pole is still a point on the surface of the Earth.

Re: North Pole

[Anonymous Coward]

Well it's certainly not the geographic north pole because that's where Robot Santa lives, and he will fuck you up.

Re:North Pole

[Swave An deBwoner]

An African or European swallow?

Re:North Pole

[wbr1]

This is not what he wants. He does not need to know 'an infinite number of points around the south pole. he other interviews check for mathematical competency and such. The answer, as is proper for any good brainteaser is in the question itself, "earth". Why, because that is your starting point, and you never went UP. It is not a technical answer he wants I'll wager.

Re:North Pole

[Dog-Cow]

The compass directions were used long before there were magnetic compasses.

Re:North Pole

[Roger W Moore]

Actual north since that is the one we use to define the lines of longitude and latitude which determine north, east, south and west and besides the north pole of the Earth's magnetic field is actually near the south pole which is why the north poles of magnets point north.

Re:North Pole

[ShanghaiBill]

I first saw this puzzle in a brainteaser book* when I was a kid, many decades ago. I think Elon's goal is to hire people that read a lot of brainteaser books, which means they are curious and have no social life, so they will make good employees. This is a smart strategy.

*The book explained that there are an infinite number of solutions: The north pole is one point, and any point 1+1/(2*pi) miles north of the south pole. He should not hire anyone that just says "north pole", since they clearly didn't read the book.

Re:North Pole

[Whiteox]

You're right. You don't need maths to solve this though. It's a thought riddle and makes some assumptions. so you ignore ice/ocean etc.
You walk South 1 mile.
Following your compass, traveling West, you're walking an arc exactly 1 mile radius from the North pole. The distance you walk West doesn't matter, you will always be 1 mile away from the pole. Now by turning North and walking 1 mile, you are back where you started.

Re:North Pole

[gordguide]

Correct, but the complete correct answer is "the geographic North Pole".

Re:North Pole

[kuzb]

No, you'd wind up in the same place because north always heads toward the pole. It doesn't matter how far you travel west or east, if you travel the same distance north as you did south from the pole, you'll wind up at the pole.

Re:North Pole

[SQLGuru]

Or a very large powered barge that is keeping your absolute position the same while you travel a relative position across its surface.......

but he's probably looking for the north pole answer.......which again, requires a rather large floating platform for at least some portion of the journey.

Re:North Pole

[saps1e]

Supposing you could walk at a speed equal to Earth's rotation (you're in really, really great shape!), the equator (or really, anywhere except very near the poles) could be an answer. Walking westward at that speed, you'd counter Earth's easterly rotation. You'd end up in a different spot in relation to the planet's surface, but be in the same position relative to space. And space is what really matters at this job, right?

Re:North Pole

[GigaplexNZ]

That violates the definition of "walk".

Re:North Pole

[ChrisMaple]

Then the question is wrong, not the answer.

Re:North Pole

[GigaplexNZ]

You're forgetting the series of points near the south pole, where walking 1 mile west does 1 or more complete laps around the pole, bringing you back to where you started.

Re:North Pole

[GigaplexNZ]

Except Earth has moved relative to the Sun, so you're not in the same position relative to space.

Re: North Pole

[Spxman]

Standing on the surface of the Earth?

Re:North Pole

[inflamed]

It also works if your name happens to be Elon.

Re:North Pole

[cyn1c77]

The north pole and a circle of lat 1 + 1 / (2 * PI) north of the south pole.

Actually the answer is the north pole and a circles of lat 1 + 1 / (2*pi*n) north of the south pole where n=1,2,3,4... etc. plus there is a slight correction because the surface of the earth is not entirely flat and so the circumference of a line of latitude is actually less than 2*pi*s where s is the arc length from the line to the south pole for the distances involved it would probably be negligible compared to surface defects.

See, if you gave the above answer, you would get a SpaceX job as an engineer due to the detailed, exact nature of your answer. Or maybe a job in their legal department.

If you just casually said "the North Pole," you would get a SpaceX job as a manager of engineers.

Re:North Pole

[theedgeofoblivious]

by Anonymous Coward

Re:North Pole

[FlyHelicopters]

Elon Musk is one of very few Billionaires who DO NOT deny that climate change is occurring

True, but you must also consider that his current primary business depends on that happening...

So the fact that he believes in AGW means nothing, he profits from it.

Re: North Pole

[swilly]

Robot Santa lives on Neptune.

Re:North Pole

[swamp_ig]

Actually there's a series of right answers around the south pole.

1+ 1/(2*pi), 1+ 1/(4*pi) , 1+ 1/(6*pi), ...

Because you can circle around the south pole any integer number of times before heading north again.

It's questionable if you can count 1 mile north of the south pole exactly, because "west" isn't defined at the south pole so would that evaluate to a no-op?

Re:North Pole

[Forever Wondering]

There are a number of southern latitute rings: See http://slashdot.org/comments.p...

Re:North Pole

[God of Lemmings]

Try Antarctica, the North Pole is often liquid.

Re:North Pole

[Gription]

Magnetic isn't irrelevant as it is both useful and more importantly for this question: It works exactly the same way.

From the magnetic north pole go magnetically south a mile, west a mile, and then north a mile and you end up at the point you started. (Unless the pole moved while you were traveling.)

Re:North Pole

[aethelrick]

if you walk from a position 2 miles away from the south pole in a southerly direction for one mile so that you are approximately 1 mile from the pole, then the distance "around the world" as you put it will be nearer three miles than one sir because a circle around the pole with a one mile radius will be 3 and a bit miles in circumference.

Re:North Pole

[jabuzz]

It also violates special and general relativity as well.

Re:North Pole

[I'm New Around Here]

Of all the clueless replies that are off the mark with regard to the "south pole solution", yours has the distinction of being the most braindead.

What is the circumference of a circle with radius r? It is (Pi * r^2), of course. So a circle with radius of 1 mile would have a circumference of (Pi * 1 * 1).
If you can't figure that out to a closer approximation than "3 and a bit miles", please turn in your Geek Card, turn off your computer, pick up a ball, and go play sports with the jocks down the road.

Re:North Pole

[Hognoxious]

Would it matter, so long as you consistently stuck to either true or magnetic directions?

Re:North Pole

[aethelrick]

I think you'll find that the previous poster didn't specify accurate measurements in miles, but rather "approximate" miles. I therefore also used an approximation of "3 and a bit" being "approximately 2 minus approximately 1" multiplied by PI. This was my point, his numbers were wrong. You'd have to start closer than two miles away from the pole to end up with a circumnavigation of a mile. With all that in mind, you sir can kiss my arse for your insulting tone and lack of good humour.

Re:North Pole

[Big Hairy Ian]

Yep I'm used to much harder interview questions than that :)

Re:North Pole

[dcw3]

And what if you cross the international dateline, and back in time?

Re:North Pole

[Talderas]

The question is very much correct. There's three tiers of answers that one can provide. The first and most trivial is the north pole. The second would be these rings of points around the south pole. The third and most correct answer is to specify one point in those rings around the south pole. The third answer demonstrates knowledge of the range of valid points while still specifying where you are.

Re:North Pole

[dcw3]

This seems an inventory question on one's ability to manipulate something 3d in the mind.

To me it seems more like a nonsense question used by an interviewer in order to make himself feel superior.

Which is why you just failed the interview. We often ask brainteaser questions, asking the interviewee to discuss their thought process out loud. We don't always expect the correct answer, but rational analysis and creativity score points.

Re:North Pole

[TheCarp]

That was my solution too, it took longer to realize it wasn't trickier than that than it did to realize the solution.

Though the solution only works if you assume the earth is a sphere and north means the where the current pole of the planets spin is and not magnetic north. However those are pretty normal assumptions for a brain teaser.

as a gauge of how easy it is I asked my wife since, she isn't someone who has done math for fun or played 1000+ hours of kerbal..... and well her answer was "Why are you asking me this? does this have a point?" Maybe I would have gotten better data after she finished her coffee?

Re:North Pole

[shmlco]

Actually, between 1 mile and 1.159 miles (.159 is the radius of a circle 1 mile in circumference).

Re:North Pole

[Hognoxious]

C = pi d = 2 pi r, so isn't it isn't it 1/(2 pi)? Then another at 1(4 pi) for two laps, 1/(6 pi) for three and so on?

Eventually they'yll be so small you'll be spinning on one foot, but theoretically they go on for ever.

Re:North Pole

[Ol Olsoc]

The answer is indeed the North Pole, and that brain teaser has been around for what, eons now?

I think I'd quickly answer it, then ask him one that I made up and tested long before that final interview.

To be annoyingly precise though, If you are at the north pole, you are not standing on earth - you are on ice.

Or some times not: http://arctic-news.blogspot.co...

Re: North Pole

[Quirkz]

Hah. I remember that one!

Re:North Pole

[605dave]

Good guess. I manage engineers!

Re:North Pole

[Minwee]

That only happens if you are walking at exactly eighty-eight miles per hour.

Re:North Pole

[Minwee]

Mars.

Re:North Pole

[Minwee]

What is the circumference of a circle with radius r? It is (Pi * r^2), of course.. So a circle with radius of 1 mile would have a circumference of (Pi * 1 * 1).

I'm just going to skip the bit about you calling someone else braindead and asking them to turn in their geek card for displaying poor math skills and leave this bit here while we move on to the bonus round.

In your own words, what is the relationship between the radius and circumference of a circle? Please be precise, as your geek card is at stake here.

Re:North Pole

[ripvlan]

I too knew the answer of North Pole - but suspected it was a trick question. My "second" guess was about relative position. If memory serves - the Earth rotates East. Therefore if you walked West at an appropriate speed you could maintain your position relative to the plane tangent to the surface. Kind of like a circus animal that walks on a large Ball or treadmill. The animal stays on top (yet moves through space) on the ball - or treadmill, you maintain a constant position in the room while the floor moves. So if you walked South --- then West -- then North you would wind up in 3D space in the original relative position.

The more I thought about this though, the speed of your walk would need to be faster during the West journey. Your North/South walk would produce a Zig shape. It isn't sqrt(2) - you trace a isosceles triangle (non-right triangle) and North/South must be the same length.

I'm ignoring the arc as 1 mile doesn't seem significant.

Re:North Pole

[dcw3]

Why not the magnetic as well, along with the multiple rings around the South Pole that also qualify.

Re:North Pole

[PrimaryConsult]

I think you're in the wrong area to be calling someone braindead...

Re:North Pole

[I'm New Around Here]

Oh my god, you're right. His braindeadedness was so strong, it sucked me into its field.

In my defense, I never did like the equation for circumference always using (2r) instead of (d), especially since in many examples the circle's diameter (eg. round swimming pool, x units across) was what was given. I saw other kids back then having this very confusion, and that was 30 years ago.

Thank you for correcting me.

Re:North Pole

[I'm New Around Here]

Hey, I hadn't had any caffeine yet. :^)

Also, see above.

Re:North Pole

[meta-monkey]

Of course then some smart ass will point out that the North Pole is covered in ice, and so is not technically by all possible definitions the surface of the earth. Somebody else in the comments made up a solution wherein it's anywhere you have a treadmill that can rotate.

Brain teasers as interview questions annoy me. Frequently because I think the interviewer (probably not Musk) is some jackass who heard Google or Microsoft gives people brain teasers, so they got a list off the internet and think they're doing their job by seeing if the applicant can get the "right" answer, which is probably the same one the applicant himself memorized before because he read the same list off the internet because they know people do this crap now, so it's all fucking pointless. If you approach it from the "let's see how this person works out a problem" thing, maybe it's valuable, but still, fuck you.

I think the best option is to make up a technically correct solution on the spot, and hope it's something original. I was once asked something like "how many quarters does it take to reach the top of the Empire State Building?" I'm guessing the clever answer is "four" as in, "four sections each 25% of the building by some measure probably height but an asshole could also say volume or mass or some dumb shit but it would still be four so who cares." Or you could try to do a Fermi estimation with the approximate height (deciding whether to call the "top" the observation deck or the highest spire) divided by the width or diameter of a US quarter dollar.

I googled ticket prices for the cheapest elevator ride to the top deck ($52) and said 214 quarters. 208 for the ticket, and then another $1.50 for a Coke when I reached the top.

(now queue some asshole telling me a Coke at the top of the Empire State Building will cost at least $3.25...)

I fucking hate brain teasers.

Re:North Pole

[athenaprime]

"Still standing on the earth."

Re: North Pole

[scum-e-bag]

I tend to see a trap as well. Admiral Ackbar is laughing at everyone here.

If we assume that the earth is a perfect sphere, then the answer is the North Pole. The earth is not a perfect sphere. This leaves us "near" the North Pole. The earths shape at the North Pole tends towards that of the Geoid as it is ice/water. As modern geodesic research has produced a good mathematical model of the Geoid, it should be possible to derive a good approximation of the actual location (or locations) with respect to the North Pole. I'd approach this problem with a recursive style of algorithm.

Please Elon, hire me.

Re:North Pole

[wallsg]

Plus any place one mile north of a set of circles pi/n miles north of the South Pole, where n is a positive integer.

Walk one mile south to a line, walk west around the circle n times, walk one mile north back to your starting point.

Re:North Pole

[wallsg]

Plus any place one mile north of a set of circles pi/n miles north of the South Pole, where n is a positive integer.

Walk one mile south to a line, walk west around the circle n times, walk one mile north back to your starting point.

Actually it's r = n/2pi. As soon as I clicked submit I realized the error.

Re:North Pole

[wallsg]

Wish I could edit.

r = 1/n2pi.

Re:North Pole

[dcw3]

Nautical or statute?

Re:North Pole

[Cederic]

If you're a mile north of the South Pole, the '1 mile west' description ceases to be possible.
If you're less than a mile north of the South Pole, the 'walk 1 mile south' description ceases to be possible.

For the scenario in the riddle, you're sure as shit nowhere near the fucking South Pole.

Re: North Pole

[BlueKitties]

Eh, my first thought was "the equator." But poles seem to work, too.

Re:North Pole

[Forever Wondering]

Yes, they do go on forever. I stopped at 100x for the sake of brevity and slashdot eccentricities (e.g. when I put only one loop per line, /. complained about "lines are too short"). At 100x, the south lat (after going one mile south) is 8 ft, 4 in (99x was 8 ft, 5 in). Eventually, the program would need to use semi-infinite precision math (e.g. double wouldn't be enough). Also, I used a 2D approximation for the distance to pole rather than calculate the arc length [I was too lazy/tired]. Still, pretty close, and good enough for illustration purposes.

#!/usr/bin/perl
# tbin/elon -- elon musk's brain teaser

#pragma pgmlns

master(@ARGV);
exit(0);

# master -- master control
sub master
{
my(@argv) = @_;

$fmt_big = "%.12f";

$M_PI = 3.14159265358979323846;
$M_PI_2 = $M_PI * 2;

# circumference of the earth
$Ce = 24901.0;

# radius of the earth
$Re = radius($Ce);

$deg_per_mile = 90.0 / $Re;

for ($loopiter = 1; $loopiter <= 100; ++$loopiter) {
dolat($loopiter);
}

{
last if ($altflg);

$bf = $out[0];
@rhs = split(" ",$bf);

$bf = title($rhs[0],"loop");
push(@title,$bf);

$bf = title($rhs[1],"degrees S lat");
push(@title,$bf);

$bf = title($rhs[2],"dist to S pole");
push(@title,$bf);

$bf = join(" ",@title);
unshift(@out,$bf);
}

foreach $bf (@out) {
print($bf,"\n");
}
}

# dolat -- calculate for single latitute
sub dolat
{
my($loopiter) = @_;

$Cx = 1.0 / $loopiter;
$Rx = radius($Cx);

# distance from southern latitude to equator
$l = $Re - $Rx;

$deg_S = ($l / $Re) * 90.0;
$deg_N = $deg_S - $deg_per_mile;

$dist_to_pole_S = $Re - $l;
$dist_to_pole_N = $dist_to_pole_S + 1;

@lhs = _showdist($dist_to_pole_N * 5280 * 12);
prtf("%3.3dx ${fmt_big} %s mi, %s ft, %s in\n",
$loopiter,$deg_N,$lhs[0],$lhs[1],$lhs[2]);
}

# _showdist -- show distance in human readable form
sub _showdist
{
my($tot,$tag) = @_;
my($tgb,@elap_tgb);
my($modlhs);
my($cur);
my($bf);
my(@lhs);

push(@elap_tgb,5280);
push(@elap_tgb,12);
push(@elap_tgb,1);

Re:North Pole

[swillden]

Frequently because I think the interviewer (probably not Musk) is some jackass who heard Google or Microsoft gives people brain teasers

Which is doubly annoying/funny because Google, at least, doesn't give people brain teasers.

Re:North Pole

[RockDoctor]

that brain teaser has been around for what, eons now?

I literally (not figuratively) cannot remember when I first heard that one, but I was using it to get a friend's head around spherical geometry in the mid-1980s, so I knew it long before then.

I'd finger Eratosthenes as the guilty party. Or a close contemporary.

Re: North Pole

[Pseudonym]

You cannot stand on the surface of the Earth at the North Pole, because it's underwater.

The correct answer... well, there are actually an infinite number of correct answers, but they're all within 1 + 1/2pi miles of the South Pole. You want a location such that walking West for one mile will take around a complete ring of latitude an integer number of times, then one mile north of there will do it. The most obvious one is the one where walking one mile will take you around the pole exactly once.

Re: North Pole

[chethanthippeswamy]

I think answer is "sitting infront of Elon musk in his space x office".

Re: North Pole

[scum-e-bag]

I'm not sure why you replied to me, the context is out place. Perhaps you are working for Elon Musk and are testing me for more info. Please, if this is the case, I am open for employment with Elon Musk and his team.

I'll make/re-itterate another point to my post at this stage though. My point relates to the common answer with regards to the South Pole answer. As I have stated, the Earth is not a perfect sphere. The shape of the Earth is known as the Geoid. If the non-spherical nature of the Earth is not taken into account, then it entirely feasible that the 1+1/2pi answer will return you to a location that is only at close proximity to your starting point. This may only be a matter of meters or fractions of meters. This sort of small error has the potential to cause an expensive rocket to explode when calculating trajectories for space flight if it is not accounted for. There are a few other factors that need to be accounted for, however, I'll let you work them out from here on in.

Re:North Pole

[scum-e-bag]

You assume that the Earth is a perfect sphere. Please see my other posts for the counter argument.

Re:North Pole

[rhodium_mir]

I would built the cone out of gold since it is so cheap now. Of course if I was a true captain of industry I would make it from that righteous metal RHODIUM. But alas, I don't have that kind of money--not even filthy paper fiat theft "money". Better send me off for processing into a fine slurry to be used as a lubricant in fracking operations and also a cheap substitute for manure.

Re: North Pole

[MemeRot]

"You're standing on the surface of the Earth." There's no reason to think that you need to be standing on dirt, just on the surface of planet Earth

Re:North Pole

[HornWumpus]

The correct answer is 'not enough information, these are the possible locations'.

When you are given a question with multiple answers on what planet is it correct to just guess at one of the possibilities.

Given there are an infinite number of answers, your guess is virtually sure to be wrong.

Re:North Pole

[Talderas]

Because choosing one of the valid locations that is not the north pole demonstrates knowledge of the "trick question" while simultaneously demonstrating decisiveness and boldness.

Re:North Pole

[cfalcon]

This is the correct answer. As long as you don't switch what you consider the north pole during your cold-ass journey, it's correct.

The bigger missing part here is the initial "one mile south". This walk is undefined- there are literally infinite ways to walk one mile south from the north pole. If you view a walk as a request to go from point A to point B, and point B is actually a 3ish mile circumference circle, you would definitely want clarification.

Re:North Pole

[niftymitch]

Or a very large powered barge that is keeping your absolute position the same while you travel a relative position across its surface.......

but he's probably looking for the north pole answer.......which again, requires a rather large floating platform for at least some portion of the journey.

He did say "on the surface of the earth" not on the surface of a barge.

However since the earth is moving at an astounding speed through
the universe and polar ice is floating and vanishing there are many footnotes
to this...

Re: North Pole

[BlueKitties]

This is a question of naming convention. You'd still be on the equator, because the equator is a circular belt around the Earth. You may be in a different location on the equator, but you'd still be on the equator. Further, Rotational mathematics is modular, so technically any direction is due south from the south pole. E.g. rotation along the complex plane from the positive real number line will always end up at the positive real number line again. Hence the roots of unity always end up back at 1, no matter how close to 1 they may start. Therefore poles is correct.

Re:North Pole

[eric_harris_76]

Or near the South Pole, one mile north of a circle a mile in circumference.

... or half a mile in circumference.

... or a third of a mile in circumference.

... etc.

Did Martin Gardner live in vain?

http://www.kyphilom.com/www/games.html#bear

At one of the poles?

[counterplex]

Lines of longitude and latitude converge into triangles at the poles. Just a guess.

Re:At one of the poles?

[Iniamyen]

Specifically the north pole, if you want to be pedantic, because "south of the south pole" would not make sense. But yes.

Re:At one of the poles?

[counterplex]

Specifically it would seem the north pole.

Re:At one of the poles?

[dfsmith]

You could be 1.2 miles north of the south pole.

Re:At one of the poles?

[HornWumpus]

North pole and anyplace on a line of latitude that is 1 + 1/(2*PI) miles north of the south pole. The 1/2*PI is an approximation that assumes the pole is flat.

I also play KSP. When do I start?

Re:At one of the poles?

[just_common_sense]

1 + (pi / 10) miles from the south pole? (1.314...) Neat idea. :-)

Re:At one of the poles?

[Penguinisto]

The distances are irrelevant truth be told, as long as the first and last legs of the trip are equal.

Re:At one of the poles?

[Daetrin]

Well that's close to the right answer. It can be the north pole, or any point one mile north of a line of latitude that is 1/[integer] miles in circumference, all of which are (very) near the south pole. Meaning that you would walk a mile south, circle the planet (at that line of latitude) exactly [integer] times, and then return one mile north to your starting point.

Re:At one of the poles?

[Anonymous Coward]

Technically, 1 + 1/(2n*PI) miles north of the south pole works, as long as "n" is a whole integer (same approximation assuming the pole is flat).

Either of the poles woulc cause this effect

[Jax Omen]

But since there's no "earth" at the north pole, the correct answer is obviously the south pole.

Re:Either of the poles woulc cause this effect

[Tumbleweed]

You can't walk South from the South Pole. :)

Re:Either of the poles woulc cause this effect

[crow]

Good luck walking one mile south from the South Pole.

Re:Either of the poles woulc cause this effect

[markdavis]

>"But since there's no "earth" at the north pole, the correct answer is obviously the south pole."

Except it can't be, because instruction #1 is to first walk 1 mile south. You can't walk south from the south pole. EVERYWHERE is north :)

So the answer is the north pole.

Re:Either of the poles woulc cause this effect

[Harodotus]

If you're standing on the south pole, it is impossible to walk a mile south. So its gotta be starting on the frozen ice at the north pole.

Re:Either of the poles woulc cause this effect

[just_common_sense]

It's impossible to walk East or West when you are at one of the poles, so it cannot be the South pole.

Re:Either of the poles woulc cause this effect

[pixelpusher220]

but you can start a mile north of the south pole (yes I looked it up!) and the solution still works.

But to be pedantic, there isn't anywhere on earth that you can currently stand on the surface of the earth that actually works. water at the north pole and way way too much ice to be on the 'surface' at the south pole :)

Re:Either of the poles woulc cause this effect

[just_common_sense]

Sorry, I was assuming you meant 1 mile from the South pole because if you were at the South pole to start, then you cannot walk South. Clearly the answer is supposed to be the North pole. (But I won't argue the "earth" part...)

Re:Either of the poles woulc cause this effect

[just_common_sense]

It doesn't work because you cannot walk East or West from the South pole.

Re:Either of the poles woulc cause this effect

[Altus]

When you walk the mile south you reach the South Pole. How do you go "west". From the South Pole every direction is north.

Re:Either of the poles woulc cause this effect

[Tumbleweed]

If you start a mile north of the South Pole, walk a mile south, then you cannot walk west, so it still fails.

Also, the North Pole isn't ice-free all year long. (I've not been keeping up with how much (if it has happened yet) it is ice-free during a year, but it's certainly not the whole year. Yet.)

Re:Either of the poles woulc cause this effect

[Guspaz]

The solution doesn't work, because your first step takes you to the south pole, and it's impossible to travel west from the south pole.

Re:Either of the poles woulc cause this effect

[NicBenjamin]

You start a more then a mile north of the South Pole. Specifically, you are 1 mile north of the latitude where there's only 1 mile of West before you're back where you started. You walk one mile South from Point A to Point B, then you walk 1 mile West and you are now back at Point B. 1 Mile North puts you back at Point A.

And I swear I figured that out before I read several dozen comments outlining that scenario already.

Re:Either of the poles woulc cause this effect

[AK Marc]

Since you are standing on a solid ground, and not on the Earth, the north pole is on the moon?

Re:Either of the poles woulc cause this effect

[idji]

how do you walk south from the south pole?

Re:Either of the poles woulc cause this effect

[xevioso]

This is the best answer here.

Re:Either of the poles woulc cause this effect

[Zero__Kelvin]

"But since there's no "earth" at the north pole, the correct answer is obviously the south pole."

That isn't correct at all. First of all, you can't walk south from the South Pole, but that doesn't even matter because:

He says the surface of the Earth. He doesn't say what that surface is made of (i.e. Ice works), or even in what millennium you are walking in (see also Plate tectonics )

Re:Either of the poles woulc cause this effect

[Zero__Kelvin]

That's great except the OP will never get your apology since you replied to yourself instead of him.

Re:Either of the poles woulc cause this effect

[Livius]

Start 1 mile + 1/(2pi) miles north of the South Pole.

There are, of course, infinitely many such points.

Re:Either of the poles woulc cause this effect

[Citizen of Earth]

You can walk on ice. Trust me; I've done it.

Re:Either of the poles woulc cause this effect

[Jack Griffin]

Start 1.2miles from the South Pole. First mile takes you to 0.2 miles north of the South Pole. The circumference of the earth at this point is roughly 1 mile (2 x pi x R - do the actual maths if you want a more accurate answer). Walk 1 mile West (or East it doesn't matter) and you end up in exactly the same spot, then walk 1 mile North retracing your steps from step 1.
The north pole also works, because ice counts as "on earth", just the same as when you walk on Ice in the south pole, in neither case are you touching actual earth.

Re:Either of the poles woulc cause this effect

[cvdwl]

There is no "earth" at the south pole. At either pole you'll be standing on about 10000' of water, whether in liquid or solid form.

Re:Either of the poles woulc cause this effect

[Megol]

[citation needed]

Re:Either of the poles woulc cause this effect

[dcw3]

This fails because you don't end up at the same point after your westward laps around the pole. The key is that your 1 mile west walk needs to end up at the same place it started. You can do one or multiple circumnavigations.

Re:Either of the poles woulc cause this effect

[dcw3]

But since there's no "earth" at the north pole, the correct answer is obviously the south pole.

The question specifically says Earth with a capital E, indicating the planet, not the soil.

I hope this is not the hardest test

[NEDHead]

they take

Re:I hope this is not the hardest test

[JTsyo]

They already have their degree, this is an interview.

similar question

[Tumbleweed]

You are in a square room. All four sides have Southern exposure. A bear walks by. What color is it? :)

Re:similar question

[Harodotus]

oh come on - a white polar bear. but then it's much more likely that if you went to the trouble to setup and build such a silly room in the first place, you could import any kind of bear you wanted with the building materials.

Re:similar question

[GTRacer]

The room or the bear?

Re:similar question

[viperidaenz]

Red.

There is also the remains of a few seals nearby.

Re:similar question

[dfsmith]

The span of the roof of my room (measured parallel to the walls) is over 13000 miles. I see brown bears. (And I've just got the bill for the tall crane that had to assemble the room exactly square. Next time, I'm aiming to see penguins!)

Re:similar question

[neminem]

Whatever color(s) you're most scared of?

Re:similar question

[kenai_alpenglow]

Since the north pole is so far from land, I'm guessing there probably aren't too many (if any) bears walking by...so I like the brown-bear solution better...

Re:similar question

[Pfhorrest]

Black, but it's covered in a bunch of transparent hairs that scatter white light something awful, obscuring its true color.

Re:similar question

[dbIII]

oh come on - a white polar bear

Given the four sides it has clearly transformed into a cartesian bear :)

Re:similar question

[dbIII]

Funny thing, those hairs block infra-red pretty well too, as discovered by a guy that stood on a polar bear while wearing night vision goggles. Luckily he also discovered he could run quite a long distance while the bear was waking up and wondering who stood on it.

Re:similar question

[dbIII]

I'm not so sure about the story of the guy stepping on one that I relayed, but this much at least is real and leads on from Pfhorrest's comment:
http://www.insidescience.org/content/new-research-reveals-how-polar-bears-stay-warm/1559

Really?

[fph il quozientatore]

This brainteaser is old and widely known. And the traditional answer is wrong. (Hint: there are an infinite number of valid points on the Earth surface, and some of the solutions are 20.000km afar one from the other.)

Re:Really?

[atfrase]

The set of points that are exactly one mile north of the south pole? I can see your reasoning except that it's hard to follow step 2 from such a starting point; how is it possible to walk one mile west once you're standing exactly on the pole?

Re:Really?

[HornWumpus]

1 + 1 / (2 * PI) north of the south pole. When you walk east you circumnavigate. They have a circumnavigation race every year at the south pole.

Re:Really?

[fph il quozientatore]

The set of points that are at distance 1+2pi/n miles from the south pole, for each positive integer n.

Re:Really?

[atfrase]

Yeah, saw your reply above. Missed that part. No SpaceX for me, I guess.

Re:Really?

[fph il quozientatore]

On a related note, there is also an infinite number of shapes a manhole cover can have so that it cannot fall into the hole. But don't tell that to the interviewers.

Re:Really?

[fph il quozientatore]

Sorry, I meant all points that are at a distance 1+1/(2pi*n) from the South pole (wrong position of the division symbol)

Re:Really?

[Registered Coward v2]

On a related note, there is also an infinite number of shapes a manhole cover can have so that it cannot fall into the hole. But don't tell that to the interviewers.

Having asked that question in interviews, I didn't care what you answer d but was interested in how you came up with answers. Answering "so they don't fall in the hole right away is the least best answer.

Re:Really?

[tommeke100]

I came up with the answer by reading books and online content about common questions asked at interviews. Do you want to know my Strengths and Weaknesses next, or just how many basketballs fit in this room?

Re:Really?

[Registered Coward v2]

I came up with the answer by reading books and online content about common questions asked at interviews.

Wrong answer. You see, when I ask that question I am wondering how well you think on your feet, not that you actually have an answer; since I really don't care about the answer you give. I need to know how you might handle an off the wall question from a client in a meeting and if I can trust you not to say something stupid. I really just want to see how you react; because you never know what a client might ask, or say.

Do you want to know my Strengths and Weaknesses next, or just how many basketballs fit in this room?

Nah, I'm more of a "How many golf balls were sold in NYC last year" kinda person.

Re:Really?

[meta-monkey]

My favorite: If Richard Feynman applied for a job at Microsoft.

Harder: self-stabilizing parachute, or balance on

[raymorris]

The harder brainteaser they SHOULD ask:

A large, cylindrical object is falling. You want it to land upright, with the correct end down. Which of these strategies do you choose:
a) Attach a parachute to the nose and let basic physics work.
b) Try to balance it atop rocket engines firing from the bottom.

Walking 3 miles?

[Anonymous Coward]

Would never do that in the first place.

Re:On the surface of the earth

[Penguinisto]

To be fair, Ice *is* a surface.

Re:Harder: self-stabilizing parachute, or balance

[dohzer]

So you're saying he should only accept people who have played a bit of Kerbal Space Program?

One obvious answer and infinite other answers.

[LawnBoy]

The obvious answer is the North Pole, but there are others. First, find the circle around the South Pole with a circumference of one mile, and then select all the points on the circle with a radius one mile larger around the South Pole. Then, find the circle around the South Pole with a circumference of one half mile, and then select all the points on the circle with a radius one mile larger around the South Pole. Then, find the circle around the South Pole with a circumference of one third mile, and then select all the points on the circle with a radius one mile larger around the South Pole. Then, find the circle around the South Pole with a circumference of one quarter mile, and then select all the points on the circle with a radius one mile larger around the South Pole. Continue ad nauseum.

Re:One obvious answer and infinite other answers.

[LawnBoy]

Your objection to the North Pole possibility relies on equivocation. You interpret "Surface of the Earth" to mean soil, but it could easily mean the surface of the planet known as Earth. If you can establish that Musk intends the interpretation you mean, then I'll throw out the North Pole, but I have no reason to assume that. Your objection to the South Pole-related answers shows that you didn't read what I wrote. I'm not saying start 1 mile from the South Pole. I said find the circle with a 1-mile circumference centered at the South Pole and start any point 1 mile North of that circle.

comment subject goes here

[Falos]

Trivia != Mental processing
/thread

Re:Harder: self-stabilizing parachute, or balance

[JWW]

b.

Congratulations, you get the job!!

Re:Harder: self-stabilizing parachute, or balance

[viperidaenz]

You also want it to land slow enough for it not to be damaged.

Still on Earth...

[gQuigs]

Relative to Mars, you might as well have not moved at all...

Re:Surprising

[mattyj]

Maybe he's not actually trying to tease your brain, but find out if you're an idiot that hasn't heard this scenario in the 200 or so years it's been floating around.

Re: *Near* the south pole...

[bill_mcgonigle]

That's right. If the story is even true, the point is likely to see how you approach it, not if you get the exact distance right. If somebody grabbed paper and pencil to work out the math and I'd asked this question that would be a serious demerit - he didn't bother checking for requirements. That's the difference between being a competent thinker and a nerd - I don't suspect SpaceX runs on nerds.

Re:Why does everyone start on the poles?

[Altus]

You are right. The answer is in the words which is why you should try reading them more carefully

Infinite Possibilities

[517714]

Aside from the North Pole: 1+n/(2 pi) miles from (North is redundant) the South Pole, where n is a positive integer.

Re:Infinite Possibilities

[517714]

1+1/2 pi n) dammit

dupe dupe dupe...

[cellocgw]

I know people don't RTFA, but apparently nobody RTFP(osts) either. we've got, what, 30 identical wrong answers (north pole only), 30 people who don't understand the difference between 1 mile from the South pole and 1mile+ X/pi ?

Just for that: imagine a Beowulf cluster of starting points in the Southern Hemisphere...

Re:dupe dupe dupe...

[SEE]

Of course, that's why it's at least theoretically a useful interview question. The North Pole answer takes just enough cognitive work to reach that upon arriving at it you can feel clever and stop. So the question filters for the people who don't stop.

(The major problem with it is that it's a reasonably famous such question; I remember reading it and learning the existence of the infinite number of South Pole answers in grade school.)

not circumnavigation, and not all straight lines

[ChipMonk]

The question says "one mile south, one mile west, and one mile north." The west-bound portion is a curved line. If you walk a straight line, after one step you are no longer walking west, but rather a south-of-west direction.

Re:Harder: self-stabilizing parachute, or balance

[Opportunist]

In that case the correct answer would be "moar boosters!"

Not the North Pole

[Jhyrryl]

The "north of the South Pole" answers are more correct...because good luck walking the open waters around the North Pole.

Re:Not the North Pole

[suutar]

your trek only takes you 1 mile from the pole. It hasn't melted that much yet.

Re:Not the North Pole

[Jhyrryl]

Dude swam it in 2007.

Re:Harder: self-stabilizing parachute, or balance

[decipher_saint]

Answer: Butter the bottom

(alt: affix cat to superstructure)

only a journalist

[sribe]

Would consider such a simple question to be a "brain teaser". In fact, I suspect this is totally fake. Why in the hell would Musk would his time and that of a freakin' rocket scientist??? He might as well demand proof that the interviewee is capable of putting on his own shoes in the morning.

Re:Harder: self-stabilizing parachute, or balance

[CAOgdin]

Good One!!!

Re:Harder: self-stabilizing parachute, or balance

[AK Marc]

http://en.wikipedia.org/wiki/L...

Rockets on the bottom is the right way to land a rocket.

The bear is white!

[mveloso]

On a treadmill using an oculus!

Correction...

[Irate Engineer]

The Brainteaser Elon Musk Used To Ask New SpaceX Engineers, Because His Old Question Got Slashdotted.

Thanks jerks!

Re:Correction...

[ShakaUVM]

>The Brainteaser Elon Musk Used To Ask New SpaceX Engineers, Because His Old Question Got Slashdotted.

I'm sure he'll just pick another one from his Big Book of Riddles for 5th Graders.

Re:Correction...

[meta-monkey]

"All right, so we've been over your history. Ph.D. in Aerospace Engineering from MIT, 12 years at the JPL. Nice. Very nice. But now we separate the men from the boys." Pulls out dog-eared copy of Big Book of Riddles for 5th Graders. "Why did the elephant paint its toenails red?"

Re:Harder: self-stabilizing parachute, or balance

[TooMuchToDo]

Difficulty level: It can't touch the water in the process.

"North Pole" is not the correct answer.

[jdharm]

The answer Musk is looking for is:

"Are these cardinal directions magnetic or geographic? Is the surface you are walking on completely flat for the entire distance? Is that 'mile' in statute miles, nautical miles, Roman miles...?"

If you pop off an answer like "North pole! Ha! That's so easy." you fail. He wants to know if you're the type of person that is going to worry about the details and isn't going to crash one of his rockets when you don't notice one software sub was calculating metric units and the other one was calculating in imperial.

Re:Harder: self-stabilizing parachute, or balance

[reve_etrange]

Maybe they want the system to work whether or not their is an atmosphere.

Better Brain Teaser

[Roger W Moore]

Agreed. A better, and more recent one, which you might nat have seen would be this one.

Re:Better Brain Teaser

[CauseBy]

Three logicians walk into a bar. The bartender says, hi guys, would you all like a drink?

The first one says, I don't know.
The second one says, I don't know.
The third one says, YES!

Re:Harder: self-stabilizing parachute, or balance

[reve_etrange]

*there

I hate puzzles in interviews

[Okian Warrior]

I really, *really* dislike hearing brain teasers in an interview.

Not because I don't like puzzles (I do), not because it's not a good way to judge the candidate (it is, in a sense), but because it shows up the deficiencies of the interviewer and the company.

Most of the time, the interviewer isn't into puzzles. They just looked something up on the internet, got a list of "here's a puzzle to ask the candidate", and mindlessly ask the question(*).

And when this happens, I answer the puzzle and then ask the interviewer my own puzzle, and see how they react.

Invariably, the answer is "I don't know. What's the answer?" within 3 seconds.

I don't want to work for someone like that, I don't want to work for a *company* that would hire someone like that, life's too short to spend time working amid thinkless drones.

A really bad company is when the VP or someone sticks his head in the door with a "hey, just wanted to see how it's going. Can you answer this question for me?" thing. I keep a chinese block puzzle in my pocket (that I invented) for this exact situation: I write down his answer on the whiteboard, hand him the puzzle, and say "if you can't disassemble this and reassemble it before the day is over, I don't want to work here".

Polite and reasonable interviews don't get this level of response, but turnabout is fair play. Ask me about my experience, ask me to solve a typical problem from the job description, get a feel for how well I work with others... these are reasonable.

But ask me why sewer caps are round, and you'll have to prove why you're company is good enough for me to work there. While you're interviewing me, I'm also interviewing *you*.

If everyone was more aggressively responsive to these types of games, companies wouldn't play them.

(*) Once, just once, I got into a real discussion of puzzles with the interviewer, I've got no problem with that. So long as it's not mindless bingo-card checkmarking, it's OK.

Re:I hate puzzles in interviews

[Jack Griffin]

I really, *really* dislike hearing brain teasers in an interview.

Not because I don't like puzzles (I do), not because it's not a good way to judge the candidate (it is, in a sense), but because it shows up the deficiencies of the interviewer and the company.

I ask puzzle questions in interviews. Not because I care about the actual answer, but I like to observe how a candidate handles something from left field. Anybody can regurgitate their resume with positive examples of their skills and experience, but not everyone can think on their feet, under pressure, and come up with some form of intelligent response (even a completely wrong one).

But ask me why sewer caps are round, and you'll have to prove why you're company is good enough for me to work there. While you're interviewing me, I'm also interviewing *you*.

That's fine, I've turned down plenty of roles too because the interviewer was terrible, but you should always be able to have a decent response for even the stupidest question. It's better to be a position to turn down a role than never be offered it. There's a lot more interviewees than interviewers, so the balance is not in your favour.

re

[JohnVanVliet]

that is an olde

i think i first heard that in the 1970's or 80's
( k 6 through 12 , sometime in there)

the north pole

but i am used to the reverse order and having the south pole as the answer

too simple. there is no bear!

[140Mandak262Jamuna]

It should be posed, "not only you are exactly where you started, there is bear trying to kill you. What color is that bear's coat?".

Re:too simple. there is no bear!

[plopez]

White. Now what sort of penguin is standing next to you?

Re:too simple. there is no bear!

[captainpanic]

Annoyed, cold and really large. Now, what nationality is the helicopter that will come and rescue your sorry ass because you went walking for 3 miles in the cold and got lost because keeping west in this case is really difficult when it is not even nearly a straight line?

Re:too simple. there is no bear!

[Whiteox]

There are no penguins at or near the North Pole.

Re:too simple. there is no bear!

[dcw3]

BZZZZT! It's red from all your blood.

Re:too simple. there is no bear!

[plopez]

You win. Imaginary would've worked as well. AC above also got it right.

Re:too simple. there is no bear!

[plopez]

Penguins don't live near the North Pole. Perhaps you should learn to use Wikipedia or Google.

Re:too simple. there is no bear!

[plopez]

Fail. There are no penguins at the North Pole.

Re:too simple. there is no bear!

[dcw3]

Fail again. "What color is that bear's coat?" Perhaps you should learn reading comprehension.

When I was developing a 2d MMORPG

[GoodNewsJimDotCom]

Back in the early 90s, I discovered the magic of seed based procedural generation to make a MMORPG world the size of an actual planet. The problem I had with a 2d based tile game is that its okay if you wrap around the edges east-west, but when if you go so far north. My solution which I never implemented was to translate you to the top of the map where you'd be translated to, but it had problems too because a player would be disoriented,"Why am I going down now?" So I was thinking you'd need to maybe flip the whole map upside down, but then that makes problems making sprites that would have to be flippable and such. This was not an easy problem to think of, but today I stick with KISS. I'd probably put them where they should be, 'maybe leave them where they're at', not flip the map, and say,"Congratulations, you just reached the top of the planet, now start going back down"

Re:When I was developing a 2d MMORPG

[General Anders]

Isn't the real problem that you can't wrap 2d based tiles into a spherical shape?

Re:When I was developing a 2d MMORPG

[JimFive]

It seems that you could model a sphere by using hexagonal tiles and generating two circular "hemispheres" that joined at the equator.
--
JimFive

Re:Harder: self-stabilizing parachute, or balance

[suutar]

and needs to land with a low enough velocity to keep it from being damaged too much.

And for the record

[Okian Warrior]

Oh, and for the record, the puzzle that Elon Musk asks is:

1) Older than dirt
2) The answer is common knowledge (hence, not a good puzzle to ask)
3) Has more than one correct answer
4) Is being asked wrong.

The actual full text of the puzzle should read something like: "A hunter walks [South... West... North... ends up in the same spot] and sees a bear. What color was the bear? (This version has only one answer.)

Here's one that *you* can ask during an interview.

You need to order weights for a 2-pan balance to weigh objects. The objects all weigh integral ounces (ie - no fractions), and the weights are all integral ounces.

What is the minimum *number of weights* you can use that lets you weigh anything up to 100 ounces?

(NB: the answer isn't 7.)

Re:And for the record

[An Ominous Coward]

Tristate for each weight: side a, side b, or neither. Ceil(log_3(max)).

Re:And for the record

[tricorn]

1, 3, 9, 27, 81

Base 3 with digits -1, 0, +1

-1 means on the same side as the object being weighed, +1 means on the opposite side. Can weigh up to 121.

Am I hired?

Re:And for the record

[JimFive]

I come up with 5:
1, 3, 9, 27, 81
To measure 73 you put 81 + 1 on one side and 9 on the other, etc

That lets you weigh up to 121. These are powers of 3 which is unintuitive to me. I looked briefly at trying to do without a 1, but there seem to be gaps e.g. with 2,3,9 you can't make 13 and you would still need 2 more to get to 100.
--
JimFive

Re:And for the record

[ZorglubZ]

You need Five. 1, 3, 9, 27 and 81oz. To weigh 2oz, you use the 1oz and the 3oz on each side; to weigh 41 you have the 81oz on one side and all the others (totalling 40oz) on the other side.

Re:And for the record

[dcw3]

Color me stumped because choosing weights based upon powers of 2 would seem to give 7 of them up to 64oz. That would allow you to weigh anything up to 127oz, which is overachieving the target, but how can the number of weights be reduced and still cover every integral ounce from 1-100?

No he doesn't...

[Anonymous Coward]

I interviewed with SpaceX for a senior-level software position last year, and was offered the job but turned it down on logistical grounds.

I did indeed have to take the tests mentioned here, and did have to interview with Musk himself as the final step. However, he did not ask me this brain teaser question. In fact, he specifically said he doesn't ask brain teaser questions because they are dumb.

Nor would he likely ask such a well-known and old brain teaser anyway. This seems like one of those things erroneously attributed to "Bill Gates" over the past 20 years because he is famous and smart, and fits people's preconceptions.

Re:No he doesn't...

[David_Hart]

I interviewed with SpaceX for a senior-level software position last year, and was offered the job but turned it down on logistical grounds.

I did indeed have to take the tests mentioned here, and did have to interview with Musk himself as the final step. However, he did not ask me this brain teaser question. In fact, he specifically said he doesn't ask brain teaser questions because they are dumb.

Nor would he likely ask such a well-known and old brain teaser anyway. This seems like one of those things erroneously attributed to "Bill Gates" over the past 20 years because he is famous and smart, and fits people's preconceptions.

This seems like a question that Musk may have asked one person one time and it became part of his story.

My Dad used to ask us kids stupid brain teasers like these. Every time we would give him the answer while rolling our eyes. Maybe he was getting us ready for a job at SpaceX. (grin)

Re:No he doesn't...

[Anonymous Coward]

He doesn't ask brain teasers because they are dumb.

That alone - that one thing - makes me want to work there. Every single software position I have applied for has been full of "look how tricksy we are, har har, he'll never figure THIS one out!" kind of time wasting trivia questions. I've turned down job offers because the company relies so heavily on this kind of an interview (I'm looking at you in particular, Amazon - and please stop sending me mail asking me to reconsider).

I've heard people defend them as, "Well, they're designed to see how someone thinks!" Even if that is the case, they're still dumb. If you want to see how someone solves problems then give them a problem that relates to the position. Don't play what is obviously a gotcha game. Yeah, I know, it takes some effort to do a proper interview and not rely on cheesy tricks like that, but that's why you have the staff you do.

Re:No he doesn't...

[nikster]

I'd have thought so.

For one, a brain teaser I can answer after about 1 second isn't really a brain teaser. Since I'm definitely not smarter than anyone who already passed through the rest of the SpaceX interview process, this would be too easy.

Second, brain teasers are stupid, I agree with Musk there. I can only imagine maybe I'm overcome by anxiety talking to Elon Musk himself, not just the CEO of a company I would like to work for, but a billionaire genius. So maybe thanks to interview anxiety I am unable to come up with the obvious answer that moment - would that make me a worse software engineer? I'm interviewing for an engineering position, not a PR gig.

Re:No he doesn't...

[techno-vampire]

That alone - that one thing - makes me want to work there. Every single software position I have applied for has been full of "look how tricksy we are, har har, he'll never figure THIS one out!" kind of time wasting trivia questions.

I remember, once, back in the early 90s, being interviewed for a position doing C programming. Part of the interview was looking at various snippits of C code and telling them what they did, just to make sure I really knew the language. I was almost stumped by one example, but finally told them that there was no way to say for sure what would happen because the outcome of that code was quite literally undefined. (Those of you who know C will know what type of thing I'm talking about.) They were quite impressed that I'd recognized this because they'd had a number of other applicants make guesses because they'd forgotten that there are some types of things that C specifically (and for very good reason) leaves undefined. I'm not sure, but that might have been what got me the job.

Re:No he doesn't...

[techno-vampire]

It was about 25 years ago, so I don't remember exactly. However, it wasn't quite as obvious as that, more on the lines of y=x/x++ where the value depends on which order the arguments are evaluated.

Re:No he doesn't...

[techno-vampire]

However your phrasing makes it sound like you have some definitive and "very clever" things in mind.

C has, among other things, the increment operator, ++, that adds one to the variable after taking the old value. (If you put the operator in front of the value, such as ++x, it increments before, rather than after taking the value.) It also specifies that the compiler can take the values of variables in whatever order is most efficient. That means that it is a Very Bad Idea to use the increment operator inside an assignment if the variable is referenced more than once because you can't know just when it gets incremented. The same goes with tests, but more so, because as soon as the answer is known, the test stops. (That is, if you're using a logical AND, if the first condition is FALSE, the second isn't checked.) That means that any "side effects," as these things are called, may or may not occur, and forgetting this Very Important Detail can cause some nasty bugs. The point of that question in the interview was to see if I remembered this.

Re:No he doesn't...

[dcollins]

I'm broadly guessing that this was the setup for a new interview question: "Were you dumb enough to believe that I ask the puzzle about walking around the poles for a job interview at SpaceX?"

Re:No he doesn't...

[rhodium_mir]

Unless it was made from intrinsically valuable rHodium, which is quite malleable.

The engineer's answer

[bl968]

Standing on a Virtuix Omni. :P http://www.virtuix.com/ (No I don't work for them in any capacity.)

Re:Harder: self-stabilizing parachute, or balance

[steveha]

Which of these strategies do you choose:
a) Attach a parachute to the nose and let basic physics work.
b) Try to balance it atop rocket engines firing from the bottom.

I realize you were going for humor (and got it; congratulations on being moderated +5 Funny). But here's a serious answer.

It depends on what you are trying to accomplish:

If your top priority is to save the rocket stage, then you pursue an engineering strategy that has the best chance of saving the rocket booster. Maybe that means a parachute system; I don't know.

But a parachute system adds mass and complexity. It becomes another critical system ("if the parachute fails, we lose the rocket stage"). The rocket stage needs functioning rocket engines, so landing on the rocket engines is another use for those engines rather than a new system with a single purpose. All else being equal, the simpler design with fewer systems is more likely to succeed in its tasks.

If you add a few hundred kilograms of parachute system mass, that's going to mean the booster can push less mass to orbit. I'd guess that the loss factor is higher than 1... that each additional kilogram of non-fuel mass on the booster reduces the to-orbit capacity by more than one kilogram. But ask a physics expert for the actual numbers.

Note that new software to make the booster land on its engines does not add mass to the booster.

So I'd say that if your top priority is to efficiently deliver stuff to orbit, the parachute system is right out and the clear engineering decision is (b).

Proving you understand spheres

[rickb928]

Glad I figured it out in about 8 seconds, including 5 seconds to check my work.

And I'm still unqualified to wok for SpaceX, which is a mild disappointment.

Re:Proving you understand spheres

[dcw3]

But, did you get all of the answers, or just the north pole?

Re: Proving you understand spheres

[rickb928]

The question 'where are you?' Have me the option of finding my one location. I didn't past that, just to avoid making the interview more complex that it needed to be.

Had they asked for the available options, I would have had to think through the others. Would I get extra points for considering 2-d options?

Re:Harder: self-stabilizing parachute, or balance

[dbraden]

Nice, nice. Started to chuckle before I finished the first sentence of the question :)

He gives you the answer

[wodencafe]

It's pretty obvious that Musk provides the answer, and then gives a pretty vital clue to the answer. "You're standing on the surface of the Earth" Now the clue is "You end up exactly where you started." Even though you've walked these miles and ended up a mile west of where you were, he tells you that you haven't gone anywhere. Because you're still standing on the surface of the earth. Simple.

My response to Elon Musk

[Okian Warrior]

And on the subject of interviewing companies, here's my response to Elon Musk:

The North Pole.

You lay a rifle on the surface of a [perfectly] spherical planet with no atmosphere. Firing the rifle, due to the curvature of the planet the bullet goes some distance and then falls to the ground. As you increase the muzzle velocity of the bullet, the point if impact gets further and further from the rifle.

If the planet has an acceleration of 10m/(s^2), what velocity must the bullet have to go around the planet and hit the gun in the stock?

(NB: This is a trick question, but Elon Musk is an actual rocket scientist.)

SpaceX guidance system

[CanEHdian]

The source for the SpaceX Falcon 9 first stage lander will win the 2015 Obfuscated C contest.

Re:Harder: self-stabilizing parachute, or balance

[Tablizer]

c) Outsource the problem to Russia.

Answers that may or may not get you the job...

[TomRC]

"I'm still right here at SpaceX, interviewing with you Elon. We only imagined that I'm walking around."
"Is there a lot of walking involved in this job? If so, you should know that I came in 2nd in a walkathon."
"Standing in a light rail car on an East/West branch of a system with stations every mile."
"Still not on Mars, damn it! Hire me and let's get going!"
"Pretty much anywhere, if the GPS in my phone malfunctions that badly."

Re:Surprising

[Austerity Empowers]

Or he's trying to hear you reason it out and doesn't care about the answer. As far as I'm concerned there are a variety of good answers to this one that all rely on one assumption or another, some answers are more righter than others. He's going to be the guy to tell you the right answer he wants anyway.

Oh that is easy.

[dsmatthews9379]

You are exactly where you where when you started. Or did he actually ask "What is the location of your starting point?" The question, as reported, is ambiguous. Otherwise it is 1 mile from the south pole. You walk to the pole, where there is no west or east, then your return north.

A brain-teaser or an honesty test?

[dpbsmith]

If I were asked that question, I think I'd answer it well. Not because I would be able to figure it out quickly under pressure, but because this brainteaser is very old.

When I was a kid in the 1950s I read both it and the original intended correct answer (the North Pole) in a book of brainteasers.

When I got into high school, someone who was actually smart discovered that the answer wasn't unique and that there was an infinite family of additional answers all involving points close to the South Pole, and I read about that, too. I'm not sure where; I think it was in Martin Gardner's "Mathematical Games" column in Scientific American.

There must be million of people who know the answer, not because they figured it out by themselves, but because they read or heard the answer somewhere.

Of all the candidates who give Mr. Musk the correct answer, I imagine very few of them are solving it on the spot. I wonder how many of the others are honest enough to volunteer the information that they had already read the answer.

Or perhaps that's the point--perhaps it's an honesty test rather than a brain-teaser.

I usually change it slightly

[CharlieG]

add "You see a bear, what color is it?"

Re:I usually change it slightly

[Anonymous Coward]

A magnificent two pound Plymoth Rock rooster sits atop a red barn with high gables oriented north-south. If it lays an egg on the peak of the barn, which side of the barn will the egg roll down?

A plane crashes on the border of North and South Dakota. Half of the passengers were American Indians, and half were just ordinary US citizens. The chief of the Standing Rock Indian reservation demands access to the crash site to recover sacred relics, and he also sues the US in federal court for burial rights. After a brief court battle, they finally come to an agreement. Where do they bury the survivors?

I've also got a few involving Elephants, refrigerators, airplanes, and bricks.

Re:I usually change it slightly

[TuringTest]

Let me guess. It involves the hunch of the elephant, the baking temperature of the refrigerator and the airspeed velocity of the unladen plane?

Miles?

[CanadianMacFan]

I'd thank him for his time and take my leave telling him that metric was the way to Mars, not that imperial crap.

Actually 2 places: North Pole and R'lyeh

[CQDX]

Anyone with half a brain can get the first answer. Anyone that I could actually work with would get the second.

Re:Actually 2 places: North Pole and R'lyeh

[delt0r]

There are technically an infinite number of places.... not just 2.

Re:Actually 2 places: North Pole and R'lyeh

[dcollins]

My understanding is that the walk at R'lyeh would be normal, but the view of the distant surroundings highly skewed, and possibly take exponential time:

http://arxiv.org/abs/1210.8144

Re:Harder: self-stabilizing parachute, or balance

[Jstlook]

I'd argue you are going to need a heck of a lot more rocket fuel for deceleration than the mass a parachute system would require.

Re:North Pole or south pole

[typo-lfm]

There are a series of spots near the south pole you can also be. Start by drawing a circle 1 mile circumference around the south pole. Start a mile outside it. also works with a circle 1/2 mile circumference or 1/3 mile or 1/4 mile and so on.

Brainteasers and Interviews

[pz]

The point of brain teasers is not to prove you're clever enough to know the answer, but to ask a question that you might not have heard before and observe your reasoning and explanations. While the North Pole question is cute, and most interviewees would know the question (at least I hope so), being able to answer it indicates not that you are smart, but that you have a certain kind of background that leads you to have been exposed to such things. Now if we continue with that assumption, then there are other questions that are worth asking.

My personal favorite question is: Explain the answer to the Monte Hall problem in such a way that a high school student could understand it.

A lot of people know the answer to the Monte Hall problem. Most people are confused by it, or get the answer wrong, but let's concentrate on those who know the answer or can figure it out on the fly. A few of them can cogently explain the reasoning behind the correct answer. Even fewer can explain it in such simple terms that a teenager could understand it. Those are the people I want to hire.

Re:Brainteasers and Interviews

[petsounds]

being able to answer it indicates not that you are smart, but that you have a certain kind of background that leads you to have been exposed to such things.

This is exactly what is wrong with software hiring, popularized in recent years by Google and thus spread throughout tech startups everywhere. You are applying inherent bias to your candidate list and self-limiting the kinds of people that could add value to the company. In other words, your hiring process will collect many people who think inside the box you want them to think in, and reject every person who thinks outside of that box. That is dangerous for both a company's intellectual and cultural health.

Re:Brainteasers and Interviews

[tricorn]

Most of the time you'll chose wrong, so you'd like to switch if only you had a clue of which one to switch to. Monty (because he already knows which one is the big prize) has conveniently given you a clue. Even though he's shown you one of the wrong doors, it's still true that your first choice was probably wrong. If your choice is probably wrong, and there's only one choice remaining, it's probably the right choice. Switch!

Re:Brainteasers and Interviews

[pz]

This is exactly what is wrong with software hiring, popularized in recent years by Google and thus spread throughout tech startups everywhere.

Really? Seems to have been a pretty winning strategy for Google. My anecdotal experience involves hiring only a small handful of people, so I wouldn't expect to draw any serious conclusions, but Google's experience surely can be used as a guide.

Ultimately, when I'm hiring a new person, I want them to be someone who likes working on solving hard problems that may or may not have solutions, and that most certainly includes thinking of new and original ways of looking at long-held beliefs. Having been exposed to brain teasers as a child is a good way of developing those skills.

a chute is simple enough for Bush. Rocket landing

[raymorris]

Well, we know that a parachute is so simple that George Bush can use it. If their design is even simpler, yet they keep screwing it up ...

There IS a reason they're trying to balance that big rocket from the bottom, but simple isn't it.

And I thought this job was for SpaceX...

[Chuckles08]

Space, people. The correct answer is, of course, in an alternate universe. Why? Because what with the Earth and our solar system flying rapidly through space, by the time you have done your 3 mile walk, you (and the Earth) will be far from where you started. Only another universe -might- happen to be in the exact spot where you started. Thank you. Elon, you can reach me on LinkedIn.

plus 1

[raymorris]

Had I not already posted in th

Re:not circumnavigation, and not all straight line

[Garfong]

The question doesn't say walking in a straight line, just walking west. A circle around the pole is the trajectory traveled when walking westward near either of the poles. The solution around the North Pole also requires walking a curved path. If you walk straight you either end up almost a mile away (if using Great Circles as your definition of "straight"), or exactly a mile away, hovering in the air (if using Euclidean "straight").

Re:not circumnavigation, and not all straight line

[rjune]

After you have traveled one mile south you would travel west on a rhumb line course. From Wikipedia, "In navigation, a rhumb line (or loxodrome) is an arc crossing all meridians of longitude at the same angle, i.e. a path with constant bearing as measured relative to true or magnetic north." In this case, you would want to measure relative to true north since the north magnetic isn't close and magnetic compasses don't work well at all at that high of a latitude. Wikipedia reference: http://en.wikipedia.org/wiki/R...

Lost ?

[slincolne]

Sorry - it's the obvious answer :-)

Re:Harder: self-stabilizing parachute, or balance

[fahrbot-bot]

b) Try to balance it atop rocket engines firing from the bottom.

(c) Balance it from rocket engines firing downward and to the sides from the top.

Re:not circumnavigation, and not all straight line

[aNonnyMouseCowered]

Mod parent up. Too sleepy to try to work this out in detail, but what struck me about most of the solutions is that it assumes a person is walking following the lines in some gigantic desktop globe. So straight lines "down" (south) and "up" (north), which should cancel out the longitudinal movement. And then a curve ball along the latitude. Would the gravitational force be strong enough at such a scale to bend the path of someone walking west into a spacetime-like curvature?

Wrong

[gerald.edward.butler]

The only correct answer is the North Pole. South means follow a line of longitude in the South direction, West means follow a line of latitude West, and North means follows a line of Longitude North. The only place this gets you back to where you started is at the North pole.

Re: Wrong

[tuck182]

The "wrong" here applies to you. This works at the north pole, but also works at an infinite number of places near the south pole, near being defined as any point a mile north of any place where one could walk a circle around the south pole that's some even fraction of a mile in diameter (one mile, half a mile, 1/3rd, etc).

Re: Wrong

[Talderas]

It does not. You failed to answer the base question, "Where are you?" If you give a range of answers, which is what you are suggesting, then you don't know where you are only where you could be.

The correct answer would be to arbitrarily select on of the points in a circle around the south pole. That demonstrates that you are aware that there are multiple answers but you at least decided where you are.

Re:Wrong

[Quirkz]

Think about it some more. There is a place near the south pole where the effective circumference of the earth at that latitude is exactly one mile. So if you started one mile north of that spot you could go south, make the circle, and head north, and be back in the same place.

Then, as the GP says, there's an infinite number, because there's also a spot where the effective circumference is half a mile, a third mile, a quarter mile, etc., where you're just doing more laps around the same ring when you head west, before heading north.

Re:Wrong

[Cederic]

No.

If you walk one mile south then you walk one mile south, not half a mile south then half a mile north.

Had the wording been, "You start walking south and continue for one mile" then I would agree with you, but it does not.

Re:Wrong

[Cederic]

Ok, no. I just realised my own misinterpretation.

Damnit.

Apologies, I blame someone else.

Canada

[Centurix]

Following a compass that is. Not sure if the magnetic north pole is still in Canadian territories as it has been wondering northward, but I'm pretty sure the magnetic south pole is well into the Indian ocean by now.

You aren't at the exact spot??

[Cutting_Crew]

one mile south, then west, then north isn't the same exact spot that you started at. You would have to go one mile east. Unless "You end up exactly where you started" means you go 1 mile east.

You're overthinking this

[Rujiel]

the question made no distinction between straight and curved lines

Re:Harder: self-stabilizing parachute, or balance

[steveha]

I'd argue you are going to need a heck of a lot more rocket fuel for deceleration than the mass a parachute system would require.

I'm not so sure of that. But I'm not an expert. Most of what I (think I) know about space I learned by reading through long discussions on the Internet.

In this case I am thinking back to discussions of SSTO craft and whether or not wings make sense; the experts all agreed that it would be better to land as a rocket than add extra mass to the system. And the key is that the thing lands almost empty... when it's taking off it's boosting itself full of fuel plus whatever upper stage(s) are in use; when it's landing it's just decelerating its own empty weight.

Balancing is a Mars practice run

[lagunastarman]

With balancing on rocket engines you get practice for landing and taking off from Mars repeatedly, if you have an orbiting "gas station" Besides, who wants to do an EVA to repack parachutes? Check out Max Hunter's RITA* concept from the 1960's if you want to see where SpaceX (and soon everyone else) is headed. *Reusable Interplanetary Transport Approach

On a treadmill

[n3r0.m4dski11z]

What do i win?

revealing south poles

[globaljustin]

if you want to be pedantic, because "south of the south pole" would not make sense

it's only a *semantic* difference...b/c of how lattitude is measured...

the *most logical* answer is near *both north or south pole* at w/e point the math says to be

it's not asking "north latitude" it just says "north"

it is an assumption based on semantics only and it is very revealing about someone's thought process

it's over-literal analysis of the question...it's a thinking error to say "only North Pole"...**however** let's note that most of the best descriptions of the why the answer works are from the over-literal "north pole only" types...

Re:revealing south poles

[globaljustin]

the *most logical* answer is near *both north or south pole* at w/e point the math says to be

i said *near* the south pole

again, a linguistic semantic distinction only

many people answered "north or south pole" and meant "near the north or south pole"

this my whole point, so much of this arguing is over semantics instead of the actual solution...which is a very revealing question, IMHO

in a competition of high-skilled almost-equals, the ability to think past semantic differences and communicate the one best answer clearly is a great way to set apart otherwise good candidates

either pole

[globaljustin]

but you can start a mile north of the south pole (yes I looked it up!) and the solution still works.

exactly...

it's only a *semantic* difference...b/c of how lattitude is measured...

the *most logical* answer is near *both north or south pole* at w/e point the math says to be

it's not asking "north latitude" it just says "north"

it is an assumption based on semantics only and it is very revealing about someone's thought process

it's over-literal analysis of the question...it's a thinking error to say "only North Pole"...**however** let's note that most of the best descriptions of the why the answer works are from the over-literal "north pole only" types...

he might as well ask about manhole covers

[YesIAmAScript]

This is the kind of dumb question which rarely if ever exposes anything about the candidate other than whether they heard this brain teaser before.

I sure hope this isn't really what he asks people.

Re:not circumnavigation, and not all straight line

[nullchar]

Gravity is the same. But to stay "west" (which is west of the pole) means you must travel in a circle, otherwise you drift south (or north at the south pole) with each straight line step.

Thanks! Re:North Pole + Near South Pole

[billstewart]

Nice to see somebody getting it right.

how come?

[PC_THE_GREAT]

How come he is exactly at same place that he was initial. If he moved 1 mile South, then 1 mile West, and then 1 mile north, he should be 1 mile west to where he initial was, dafuq buggy question. To reach the initial position there should have been a 1 mile east associated to negate the west movement.

Re:how come?

[PC_THE_GREAT]

*initially :)

Re:North Pole (South pole ring data)

[Forever Wondering]

Here's the first hundred latitutes [I wrote a perl program--so it could never be wrong ;-) and I had to derive it]

loop degrees S lat dist to S pole
001x 89.973676291007 1 mi, 840 ft, 4 in 002x 89.975483447346 1 mi, 420 ft, 2 in
003x 89.976085832792 1 mi, 280 ft, 1 in 004x 89.976387025515 1 mi, 210 ft, 1 in
005x 89.976567741149 1 mi, 168 ft, 0 in 006x 89.976688218238 1 mi, 140 ft, 0 in
007x 89.976774273302 1 mi, 120 ft, 0 in 008x 89.976838814600 1 mi, 105 ft, 0 in
009x 89.976889013387 1 mi, 93 ft, 4 in 010x 89.976929172417 1 mi, 84 ft, 0 in
011x 89.976962029805 1 mi, 76 ft, 4 in 012x 89.976989410962 1 mi, 70 ft, 0 in
013x 89.977012579633 1 mi, 64 ft, 7 in 014x 89.977032438493 1 mi, 60 ft, 0 in
015x 89.977049649506 1 mi, 56 ft, 0 in 016x 89.977064709142 1 mi, 52 ft, 6 in
017x 89.977077997057 1 mi, 49 ft, 5 in 018x 89.977089808536 1 mi, 46 ft, 8 in
019x 89.977100376702 1 mi, 44 ft, 2 in 020x 89.977109888051 1 mi, 42 ft, 0 in
021x 89.977118493557 1 mi, 40 ft, 0 in 022x 89.977126316745 1 mi, 38 ft, 2 in
023x 89.977133459655 1 mi, 36 ft, 6 in 024x 89.977140007323 1 mi, 35 ft, 0 in
025x 89.977146031178 1 mi, 33 ft, 7 in 026x 89.977151591659 1 mi, 32 ft, 3 in
027x 89.977156740252 1 mi, 31 ft, 1 in 028x 89.977161521089 1 mi, 30 ft, 0 in
029x 89.977165972213 1 mi, 28 ft, 11 in 030x 89.977170126595 1 mi, 28 ft, 0 in
031x 89.977174012953 1 mi, 27 ft, 1 in 032x 89.977177656414 1 mi, 26 ft, 3 in
033x 89.977181079058 1 mi, 25 ft, 5 in 034x 89.977184300371 1 mi, 24 ft, 8 in
035x 89.977187337608 1 mi, 24 ft, 0 in 036x 89.977190206110 1 mi, 23 ft, 4 in
037x 89.977192919558 1 mi, 22 ft, 8 in 038x 89.977195490193 1 mi, 22 ft, 1 in
039x 89.977197929001 1 mi, 21 ft, 6 in 040x 89.977200245868 1 mi, 21 ft, 0 in
041x 89.977202449717 1 mi, 20 ft, 5 in 042x 89.977204548621 1 mi, 20 ft, 0 in
043x 89.977206549902 1 mi, 19 ft, 6 in 044x 89.977208460215 1 mi, 19 ft, 1 in
045x 89.977210285625 1 mi, 18 ft, 8 in 046x 89.977212031670 1 mi, 18 ft, 3 in
047x 89.977213703415 1 mi, 17 ft, 10 in 048x 89.977215305504 1 mi, 17 ft, 6 in
049x 89.977216842202 1 mi, 17 ft, 1 in 050x 89.977218317431 1 mi, 16 ft, 9 in
051x 89.977219734809 1 mi, 16 ft, 5 in 052x 89.977221097672 1 mi, 16 ft, 1 in
053x 89.977222409106 1 mi, 15 ft, 10 in 054x 89.977223671968 1 mi, 15 ft, 6 in
055x 89.977224888909 1 mi, 15 ft, 3 in 056x 89.977226062387 1 mi, 15 ft, 0 in
057x 89.977227194690 1 mi, 14 ft, 8 in 058x 89.977228287949 1 mi, 14 ft, 5 in
059x 89.977229344148 1 mi, 14 ft, 2 in 060x 89.977230365140 1 mi, 14 ft, 0 in
061x 89.977231352657 1 mi, 13 ft, 9 in 062x 89.977232308319 1 mi, 13 ft, 6 in
063x 89.977233233642 1 mi, 13 ft, 4 in 064x 89.977234130049 1 mi, 13 ft, 1 in
065x 89.977234998874 1 mi, 12 ft, 11 in 066x 89.977235841371 1 mi, 12 ft, 8 in
067x 89.977236658719 1 mi, 12 ft, 6 in 068x 89.977237452028 1 mi, 12 ft, 4 in
069x 89.977238222342 1 mi, 12 ft, 2 in 070x 89.977238970646 1 mi, 12 ft, 0 in
071x 89.977239697872 1 mi, 11 ft, 10 in 072x 89.977240404898 1 mi, 11 ft, 8 in
073x 89.977241092552 1 mi, 11 ft, 6 in 074x 89.977241761622 1 mi, 11 ft, 4 in
075x 89.977242412849 1 mi, 11 ft, 2 in 076x 89.977243046939 1 mi, 11 ft, 0 in
077x 89.977243664559 1 mi, 10 ft, 10 in 078x 89.977244266343 1 mi, 10 ft, 9 in
079x 89.977244852891 1 mi, 10 ft, 7 in 080x 89.977245424776 1 mi, 10 ft, 6 in
081x 89.977245982541 1 mi, 10 ft, 4 in 082x 89.977246526701 1 mi, 10 ft, 2 in
083x 89.977247057749 1 mi, 10 ft, 1 in 084x 89.977247576153 1 mi, 10 ft, 0 in
085x 89.977248082359 1 mi, 9 ft, 10 in 086x 89.977248576793 1 mi, 9 ft, 9 in
087x 89.977249059861 1 mi, 9 ft, 7 in 088x 89.977249531950 1 mi, 9 ft, 6 in
089x 89.977249993430 1 mi, 9 ft, 5 in 090x 89.977250444655 1 mi, 9 ft, 4 in
091x 89.977250885963 1 mi, 9 ft, 2 in 092x 89.977251317677 1 mi, 9 ft, 1 in
093x 89.977251740108 1 mi, 9 ft, 0 in 094x 89.977252153550 1 mi, 8 ft, 11 in
095x 89.977252558288 1 mi, 8 ft, 10 in 096x 89.977252954594 1 mi, 8 ft, 9 in
097x 89.977253342729 1 mi, 8 ft, 7 in 098x 89.977253722943 1 mi, 8 ft, 6 in
099x 89.977254095476 1 mi, 8 ft, 5 in 100x 89.977254460558 1 mi, 8 ft, 4 in

He botched it.

[jcr]

The way this goes is: A hunter walks one mile south, then one mile west, and he shoots a bear. He then walks one mile north, and arrives at his starting point. What color was the bear?

Answer: it was white, because the north pole is the only place where the movement described is possible.

-jcr

Re:Harder: self-stabilizing parachute, or balance

[delt0r]

Or strap a cat somewhere to the superstructure....

Re:not circumnavigation, and not all straight line

[dcw3]

The north and south bound portions are curved lines as well, unless you want to become airborne.

Re:Harder: self-stabilizing parachute, or balance

[daq man]

You also have :

c) Attach a ring of rockets to the top of the cylinder and allow gravity to balance it for you.

d) Use a teardrop shape instead of a cylinder so that it falls blunt end first, add fold out wings at the other end to produce drag and use the rockets at the top to slow the thing down.

e) Forget the land upright part, stick fold out wings on it and land it horizontally like an aircraft.

Physics Nerd Jokes

[DarthVain]

1) First assume that the Earth is a perfect sphere
2) ???
3) PROFIT!!!

Citation:
http://en.wikipedia.org/wiki/S...

Re:not circumnavigation, and not all straight line

[Ol Olsoc]

"In navigation, a rhumb line (or loxodrome) is an arc crossing all meridians of longitude at the same angle

Don't be silly - a loxodrome is a building where they raise tasty fish.

That's it?

[rnturn]

I knew the answer to that one in grade school. I hope that successfully answering that question is not a major factor in getting hired there.

Earth

[I didn't]

n/t

Re:How is the north and south pole more round?

[SecurityGuy]

I think you found the worst possible answer to this question.

"Elon, I finished the task you gave me! I haven't actually done what you wanted, I just redefined the terms so I was done before I started."

I usually say asking such questions in an interview is a terrible idea, but I'd honestly disqualify anyone who gave an answer like this.

three spheres

[RSilverlok]

Draw two spheres about 3/4 overlapped , now draw a third sphere that completely surrounds the first two. You now have an accurate Ven diagram of Theory, Mathematical modeling, and Reality. The big circle is reality. And for the riddle the devil, as always, is in the details. One cannot stand on the earth at the north pole. Also triangles on spheres are cool but and equilateral triangle on an oblate spheroid isn't really possible , even at the north pole . The only reasonable answer where One could stand on the earth and walk an equilateral triangle right back to their starting point with a fair chance of success would be someplace really flat , so my guess would be that I am at : The salt flats in Bolivia, Salar de Uyuni

Re: musk teaser

[dcw3]

BZZZZZ! Close but no cigar. You should have gone for 1 mile in circumference instead of diameter. There are also many other circles which would cause you to take 2, 3, 4.... laps around the pole before heading north.

Yeah..

[richieb]

And then you shoot a bear. What color is the dear?

Interview Question

[brunnegd]

I asked prospective engineering hires this question: I own a major league ballpark. I need new grass. If turf is $1/square ft., about how much would it cost me to resod the field? I didn't care about the answer, I wanted to see their thinking. I was amazed at how many couldn't even start to work the problem, many said they needed their computer to start.

Incorrectly phrased question!

[doccus]

Re: "...You walk one mile south, one mile west, and one mile north. You end up exactly where you started. Where are you?" Nowhere in the question is the proviso "after which" or any condition of not using transport other than walking stated... as in "...one mile west, and one mile north AFTER WHICH you end up exactly where you started. Where are you?" So, as this was not stated, I could have hopped a plane from the landing point back to the start point. Not using transport might be "implied" in the question , but that's only a personal matter of interpretation. It's not actively forbidden. Furthermore, the question says "You're standing on the surface of the earth", in which could be argued that it is immediately ruling out the north pole, unless, of course, ICE is considered "the surface of the earth" . A geologist might be the one to ask this question to, if Ice is "the surface".. I suspect that this more lengthy response might be part of the answer Musk expected, because the "north pole" answer is so patently obvious that it, by itself , proves nothing intelligence wise !

Re:Harder: self-stabilizing parachute, or balance

[cmdr_klarg]

Answer: Butter the bottom

(alt: affix cat to superstructure)

Do both for redundancy

The bear is white.

[MondoGordo]

The "correct" answer depends on the purpose of the question ...

What does Elon really want to know ?

1) Do you think outside the box ? - North Pole is the wrong answer.

2) Are you a math geek ? - North Pole is the wrong answer.

3) Are you the sort who recognizes that the lack of a unique answer and protests the fairness of the question? - North Pole is probably still the wrong answer

4) Will you embrace the KISS principle? - North Pole is the RIGHT answer.

The form of the teaser that I originally heard is ... "You're standing on the surface of the Earth. You walk one mile south, one mile west, and one mile north. You end up exactly where you started and you see a bear. What colour is the bear?"

Re:Harder: self-stabilizing parachute, or balance

[jfengel]

But it does require extra fuel. I'd have expected that fuel to be more than the weight of a parachute system, though perhaps not: it would be lowering a mostly-empty tin can.

I imagine that it's a bonus to be able to have that kind of precision on your rocket engines: if you can get them down, then it may provide advantages in going up. Certainly it's nice that you've proven that kind of control.

Logically..

[jcfigueiredo]

You are on the surface of the Earth, which it's where you've started. - focus on the question, not on the solution.

other families of solutions

[rlseaman]

There are other ways of looking at the problem:

"You're standing on the surface of the Earth. You walk one mile south, one mile west, and one mile north. You end up exactly where you started. Where are you?"

Other posters have pointed out that "Where are you?" is ambiguous and could mean a Simon says sort of answer like "I'm in your office, Mr. Musk." And also that it could be taken to mean relative to a Sun-centered coordinate system. This latter requires waiting N years to return to the same part of the Earth's orbit.

Once one notices that no time limit is required you get many more solutions by allowing for the polar motion over some period of time: http://en.wikipedia.org/wiki/P... - that is, the pole isn't in the same place at the end as at the beginning.

Then there's the notion of repeating the exercise at the north and south magnetic poles (and perhaps geomagnetic). But there is also no explicit constraint that south, west and north are all interpreted the same - they need not all be geographic or all magnetic. In that case there are families of solutions near each of the four poles that interpret the initial motion one way and the final mile the other.

And then the magnetic poles wander much more rapidly (several miles per year) than the geographic poles and over much more than the mile allowed (http://en.wikipedia.org/wiki/South_Magnetic_Pole), so you can put constraints on the time period allowed for the exercise while exploring solutions covering motions over a few weeks.

Re:other families of solutions

[rlseaman]

...and different Geodetic datums ...and continental drift

North pole?

[Rainwulf]

How hard is that.. seriously.

Re:Harder: self-stabilizing parachute, or balance

[reve_etrange]

Well there is bound to be some shit floating around any gravity well. As long as we are talking spherical cows in a vacuum just make the massless parachute larger? How do you actually define "atmosphere" anyways?

Say we want to land on Mars. The atmosphere is 100 times less dense than on Earth, so we'll need 100 times more parachute. I'm sure that's still a lot less mass and complexity than the balancing rockets...but what if we want to make a round trip, and need to land on Mars and Earth?

We're going to need parallel parachute systems of different sizes, or a parachute that can be re-packaged combined with variable unfurling, and suddenly the parachute option is looking a lot more complicated. On the other hand, if you get the balancing rockets to work, you can use them anywhere, over and over again.

An infinite number of possible answers

[sbaker]

Consider this...suppose you are just over a mile from the SOUTH pole. You walk a mile south - and now you're maybe a hundred feet from the South pole. Then you turn west and start walking...around and around in a tiny 100 foot radius circle centered on the pole. When you've finally clocked up a mile - you turn and head North again...where do you end up?

Well, the answer depends on the exact circumference of the circle that you walked around. Generally, you'll end up someplace very different from your starting point...BUT if that circle is an EXACT sub-multiple of a mile - then you'll end up precisely where you started.

So...the North pole is clearly NOT a unique answer.

Furthermore - the north pole is only ONE answer. My approach reveals an infinite number of possible answers:

1) You could have started ANYWHERE that's at the exact right distance from the pole - so anywhere on that circle will do...an infinite number of starting points will work.

2) Note that ANY exact sub-multiple of a mile will do - so with mathematical precision, there are an infinite number of sub-multiples of a mile - and hence an infinite number of distances from the pole where you could have started.

Truly - the "North Pole" example exhibits very little lateral thinking... if that was your answer then you **FAILED** the Musk test...which (I'm pretty sure) is the whole point here.

The original version of the story is that a hunter walk a mile south, a mile west, shoots a bear, then walks a mile north to return to his starting point. What color was the bear?

Since there are no bears at the south pole - and only polar bears live anywhere near the north pole - then the north pole is the right place and the correct answer is "WHITE!"....but Musk isn't asking *that* question...he's trying to trick people into jumping to a false conclusion without stopping to think about it.

    -- Steve Baker

Re:An infinite number of possible answers

[scum-e-bag]

You assume that the Earth is a perfect sphere. See my other posts for the counter argument.

Frame of reference

[AdeBaumann]

I think it depends on your frame of reference. North Pole and the various rings around the south pole are correct if you want to end up in the same location with reference to the earth. If you observe the situation from the sun, there's no possible point, since the earth moves while you walk. If you observe the situation with yourself as the frame of reference, any point will do, since you will always end up where you are...

Re:not circumnavigation, and not all straight line

[cfalcon]

EVERY west-bound portion is a curved line.

same place?

[umop apisdn]

the simplest answer is "still on the surface of the earth," of course. everything else is an overreaction based on irrelevant information.

Maybe a different test?

[javakcl]

Maybe testing to see if the person tends to over-complicate things?