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The Brainteaser Elon Musk Asks New SpaceX Engineers

Posted by ryuzaki0 on from the riddle-me-this dept.

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.

50 of 496 comments (clear)

  1. North Pole by 605dave · · Score: 4, Informative

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

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    1. Re:North Pole by Jamu · · Score: 5, Funny

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

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      Who ordered that?
    2. Re:North Pole by Penguinisto · · Score: 3, Informative

      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.

      --
      Quo usque tandem abutere, Nimbus, patientia nostra?
    3. Re:North Pole by HornWumpus · · Score: 2, Insightful

      You all fail.

      --
      John McAfee 'It was like that time I hired that Bangkok prostitute; to do my taxes, while I fucked my accountant'
    4. Re:North Pole by abelenky17 · · Score: 5, Insightful

      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.

    5. Re:North Pole by HornWumpus · · Score: 4, Insightful

      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.

      --
      John McAfee 'It was like that time I hired that Bangkok prostitute; to do my taxes, while I fucked my accountant'
    6. Re:North Pole by softarch · · Score: 5, Informative

      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....

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      Apply your own interpretation of the words and grammar in this post.
    7. Re:North Pole by IcyWolfy · · Score: 4, Informative

      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)

    8. Re:North Pole by Roger+W+Moore · · Score: 5, Informative

      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.

    9. Re:North Pole by Anonymous Coward · · Score: 2, Insightful

      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.

    10. Re:North Pole by 605dave · · Score: 2

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

      --
      Be kind, for everyone you meet is fighting a difficult battle. - Plato
    11. Re:North Pole by Tablizer · · Score: 2

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

      --
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    12. Re:North Pole by Swave+An+deBwoner · · Score: 2

      An African or European swallow?

    13. Re:North Pole by kuzb · · Score: 2

      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.

      --
      BeauHD. Worst editor since kdawson.
    14. Re:North Pole by cyn1c77 · · Score: 4, Funny

      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.

    15. Re:North Pole by God+of+Lemmings · · Score: 2

      Try Antarctica, the North Pole is often liquid.

      --
      Non sequitur: Your facts are uncoordinated.
    16. Re:North Pole by jabuzz · · Score: 2

      It also violates special and general relativity as well.

    17. Re:North Pole by Minwee · · Score: 3, Funny

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

  2. Either of the poles woulc cause this effect by Jax+Omen · · Score: 2

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

    1. Re:Either of the poles woulc cause this effect by just_common_sense · · Score: 3, Informative

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

    2. Re:Either of the poles woulc cause this effect by NicBenjamin · · Score: 2

      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.

  3. Harder: self-stabilizing parachute, or balance on by raymorris · · Score: 5, Funny

    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.

  4. Re:similar question by Harodotus · · Score: 2

    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.

    --
    Its not users who are broken, it's systems not taking account their likely behaviour and fixing it technically.
  5. Re:similar question by GTRacer · · Score: 4, Funny

    The room or the bear?

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  6. Re:At one of the poles? by HornWumpus · · Score: 2

    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?

    --
    John McAfee 'It was like that time I hired that Bangkok prostitute; to do my taxes, while I fucked my accountant'
  7. Re:On the surface of the earth by Penguinisto · · Score: 2

    To be fair, Ice *is* a surface.

    --
    Quo usque tandem abutere, Nimbus, patientia nostra?
  8. One obvious answer and infinite other answers. by LawnBoy · · Score: 3, Informative

    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.

  9. Re:Really? by fph+il+quozientatore · · Score: 4, Interesting

    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.

    --
    My first program:

    Hell Segmentation fault

  10. Re:Harder: self-stabilizing parachute, or balance by Opportunist · · Score: 2

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

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    We used to have a Bill of Rights. Now, with the rights gone, all we have left is the bill.
  11. Re:Harder: self-stabilizing parachute, or balance by decipher_saint · · Score: 4, Funny

    Answer: Butter the bottom

    (alt: affix cat to superstructure)

    --
    crazy dynamite monkey
  12. Correction... by Irate+Engineer · · Score: 5, Funny

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

    Thanks jerks!

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    1. Re:Correction... by ShakaUVM · · Score: 2

      >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.

    2. Re:Correction... by meta-monkey · · Score: 3

      "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?"

      --
      We don't have a state-run media we have a media-run state.
  13. Re:Harder: self-stabilizing parachute, or balance by reve_etrange · · Score: 4, Interesting

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

    --
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  14. Better Brain Teaser by Roger+W+Moore · · Score: 2

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

    1. Re:Better Brain Teaser by CauseBy · · Score: 2

      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!

  15. No he doesn't... by Anonymous Coward · · Score: 5, Informative

    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.

    1. Re:No he doesn't... by techno-vampire · · Score: 3, Interesting

      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.

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    2. Re:No he doesn't... by techno-vampire · · Score: 2

      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.

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  16. I usually change it slightly by CharlieG · · Score: 2

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

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  17. Miles? by CanadianMacFan · · Score: 2

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

  18. Actually 2 places: North Pole and R'lyeh by CQDX · · Score: 2

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

  19. Re:Harder: self-stabilizing parachute, or balance by fahrbot-bot · · Score: 2

    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.

    --
    It must have been something you assimilated. . . .
  20. Re:similar question by Pfhorrest · · Score: 2

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

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  21. Re: Wrong by tuck182 · · Score: 2

    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).

  22. Balancing is a Mars practice run by lagunastarman · · Score: 2

    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

  23. Re:similar question by dbIII · · Score: 4, Funny

    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.

  24. He botched it. by jcr · · Score: 2

    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

    --
    The only title of honor that a tyrant can grant is "Enemy of the State."
  25. Re:not circumnavigation, and not all straight line by Ol+Olsoc · · Score: 2

    "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.

    --
    The shepherds did so well protecting the flock that the sheep no longer believed that wolves existed.
  26. Re:How is the north and south pole more round? by SecurityGuy · · Score: 2

    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.