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

[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

[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

[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

[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

[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

[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

[Minwee]

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

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.

Re:similar question

[GTRacer]

The room or the bear?

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.

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: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:Harder: self-stabilizing parachute, or balance

[decipher_saint]

Answer: Butter the bottom

(alt: affix cat to superstructure)

Correction...

[Irate Engineer]

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

Thanks jerks!

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

[reve_etrange]

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

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...

[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: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.