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.

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:

[HornWumpus]

You all fail.

Re:

[Penguinisto]

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

How is that failure?

Re:

[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.

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:

[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.

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:

[Swave An deBwoner]

An African or European swallow?

Re:

[viperidaenz]

Not eons, Elons

Better Brain Teaser

[Roger W Moore]

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

Re:

[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: 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: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

[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:

[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:

[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

[Minwee]

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

Re:

[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:

[jabuzz]

It also violates special and general relativity as well.

Re:

[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 br

Re:

[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:

[605dave]

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

Re:

[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:

[God of Lemmings]

Try Antarctica, the North Pole is often liquid.

Re:

[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.

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:

[Tumbleweed]

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

Re:

[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:

[just_common_sense]

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

Re:

[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:

[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:

[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:

[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:

[crow]

Good luck walking one mile south from the South Pole.

Re:

[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:

[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:

[AK Marc]

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

Re:

[idji]

how do you walk south from the south pole?

I hope this is not the hardest test

[NEDHead]

they take

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:

[dohzer]

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

Re:

[Opportunist]

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

Re:

[JWW]

b.

Congratulations, you get the job!!

Re:

[viperidaenz]

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

Re:Harder: self-stabilizing parachute, or balance

[decipher_saint]

Answer: Butter the bottom

(alt: affix cat to superstructure)

Re:

[CAOgdin]

Good One!!!

Re:

[AK Marc]

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

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

Re:

[TooMuchToDo]

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

Re:Harder: self-stabilizing parachute, or balance

[reve_etrange]

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

Re:

[reve_etrange]

*there

Re:

[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.

similar question

[Tumbleweed]

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

Re:

[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:

[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

[GTRacer]

The room or the bear?

Re:

[viperidaenz]

Red.

There is also the remains of a few seals nearby.

Re:

[neminem]

Whatever color(s) you're most scared of [wikipedia.org]?

Re:

[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]

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.

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:

[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:

[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:

[atfrase]

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

Re:

[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:

[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?

[fph il quozientatore]

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

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.

comment subject goes here

[Falos]

Trivia != Mental processing
/thread

Still on Earth...

[gQuigs]

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

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:

[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...

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.

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:

[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?"

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 inte

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:

[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 operato

I usually change it slightly

[CharlieG]

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

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.

Comment removed

[account_deleted]

Comment removed based on user account deletion

Re:

[Iniamyen]

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

Re:

[dfsmith]

You could be 1.2 miles north of the south pole.

Re:

[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:

[Penguinisto]

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

Re:

[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:

[Penguinisto]

To be fair, Ice *is* a surface.

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:

[Altus]

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

Re:

[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.