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The Brainteaser Elon Musk Asks New SpaceX Engineers 496

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.
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The Brainteaser Elon Musk Asks New SpaceX Engineers

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  • North Pole (Score:5, Informative)

    by 605dave ( 722736 ) on Wednesday May 20, 2015 @05:35PM (#49739775) Homepage

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

    • by Jamu ( 852752 ) on Wednesday May 20, 2015 @05:41PM (#49739849)
      or a treadmill, but you'd have to turn it 90 degrees clockwise twice.
    • Re:North Pole (Score:4, Informative)

      by Penguinisto ( 415985 ) on Wednesday May 20, 2015 @05:41PM (#49739873) Journal

      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: (Score:3, Insightful)

        by HornWumpus ( 783565 )

        You all fail.

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

          How is that failure?

        • by 605dave ( 722736 )

          Then what's the answer?

          • Re:North Pole (Score:5, Insightful)

            by HornWumpus ( 783565 ) on Wednesday May 20, 2015 @05:48PM (#49739967)

            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.

            • 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:North Pole (Score:5, Informative)

              by Roger W Moore ( 538166 ) on Wednesday May 20, 2015 @06:11PM (#49740205) Journal

              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.

              • by cyn1c77 ( 928549 ) on Thursday May 21, 2015 @01:36AM (#49741661)

                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.

      • Not eons, Elons

      • Agreed. A better, and more recent one, which you might nat have seen would be this one [theguardian.com].
        • 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:North Pole (Score:5, Insightful)

      by abelenky17 ( 548645 ) on Wednesday May 20, 2015 @05:45PM (#49739933)

      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 (Score:5, Informative)

        by softarch ( 661894 ) on Wednesday May 20, 2015 @05:50PM (#49740003)
        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....
      • 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.

    • 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 (Score:5, Informative)

      by IcyWolfy ( 514669 ) on Wednesday May 20, 2015 @05:55PM (#49740055) Homepage

      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)

    • 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 br
    • 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.
    • by msauve ( 701917 )
      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.
    • Try Antarctica, the North Pole is often liquid.
  • by Jax Omen ( 1248086 ) on Wednesday May 20, 2015 @05:35PM (#49739785)

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

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

      • 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: (Score:3, Informative)

          It doesn't work because you cannot walk East or West from the South pole.
        • Re: (Score:2, Insightful)

          by Altus ( 1034 )

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

          • 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.

        • 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.)

        • by Guspaz ( 556486 )

          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.

    • by crow ( 16139 )

      Good luck walking one mile south from the South Pole.

    • >"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.

    • 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.
    • by AK Marc ( 707885 )
      Since you are standing on a solid ground, and not on the Earth, the north pole is on the moon?
    • by idji ( 984038 )
      how do you walk south from the south pole?
  • You are in a square room. All four sides have Southern exposure. A bear walks by. What color is it? :)

  • 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.)
  • by LawnBoy ( 858717 ) on Wednesday May 20, 2015 @05:47PM (#49739949)
    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.
  • Trivia != Mental processing
    /thread
  • Relative to Mars, you might as well have not moved at all...

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

  • 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.

  • On a treadmill using an oculus!

  • by Irate Engineer ( 2814313 ) on Wednesday May 20, 2015 @06:14PM (#49740235)

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

    Thanks jerks!

    • >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.

  • 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 inte

  • No he doesn't... (Score:5, Informative)

    by Anonymous Coward on Wednesday May 20, 2015 @06:39PM (#49740395)

    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.

  • by CharlieG ( 34950 ) on Wednesday May 20, 2015 @08:31PM (#49740725) Homepage

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

  • by CanadianMacFan ( 1900244 ) on Wednesday May 20, 2015 @08:34PM (#49740729)

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

  • by CQDX ( 2720013 ) on Wednesday May 20, 2015 @08:36PM (#49740731)

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

  • by jcr ( 53032 ) <jcr@mac.cOPENBSDom minus bsd> on Thursday May 21, 2015 @05:29AM (#49742189) Journal

    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

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