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Comment by seanmcdirmid | Hacker News Reader

It seems like one of those stupid brain teasers tech companies used to ask that are not very correlated with success but act as arbitrary filters.

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In this case the "arbitrary filter" was filtering applicants to a university for very basic math knowledge, so I think assuming that it's correlated with success at that university is not very far fetched.

Euclidean geometry games are hardly what people do in the real world, but if it floats your boat:

https://www.euclidea.xyz

I don't think after playing 20 or so levels I was much smarter for it. It turned out to be a bunch of rules that were extremely limited. You do begin to appreciate trig much more.

If you are being given a geometry subject test and are prepared for it, this is fair game. If the question is just given out of the blue as a brain teaser, then no, the university is not mining useful signals at all.

Thank you. Here goes the 2 or so hours of me doodling away.. :)

Sure, but it also filters for people like me, who have seen this problem at least a dozen times.

Ya, these also can totally be gamed with the right amount of prep.

So why doesn't everyone pass the interview?

Either they can't be gamed, or they select for people who can at least be bothered to do the work to game them.

Depends on your resources for prep. Poor kids have less resources for that than rich kids, so they ultimately select for richer candidates.

When I did my university entrance interview (for maths, at Cambridge) the interviewers were clear that they expected that some subset of candidates would have seen the problem before, and some wouldn't -- for those in the first set they'd get them to quickly go through the problem and move onto the later parts which would be new to them; for those in the second set they'd provide sufficient guidance to let the candidate walk through the problem. The point was to get any particular candidate to a point in the problem sequence where this was something new to them, and then see how they tackled things. The idea that some applicants (usually from public schools) would have been very highly prepped for interview and others (usually from state schools) would not was clearly something they were well aware of and setting their interview design up to handle.

But how do you know it's a new problem? You can always fake a little struggle and thinking your way to the solution for a problem that you know the answer already.

Yes, because they don't want to measure "success" but knowledge of basic math.

Which is par for the course for an educational institution (and a business one could argue).

I don't see anything tricky about that problem. It requires some junior high geometry and high school trig, and yes, I suppose you need to know the radius of the earth offhand. Still. That strikes me a lot more as a math fizzbuzz than as a brain teaser. A college applicant can be reasonably expected to know this stuff.

I suppose you need to know the radius of the earth offhand.

Or you can easily derive the radius if you remember that a meter was initially defined as 1/10,000,000 of the distance from the equator to the North Pole. So you can easily calculate the Earth's circumference, and from there the radius: 10,000,000 * 4 / 1000 / 3.1416 / 2 = 6366 km.

Wikipedia gives 6371 km as the mean radius.

It also seems to be a less coherent version of the chicken from Minsk puzzle.

Perhaps it's multiple choice.

  A.  Dreadfully sorry, but I'm afraid I haven't memorized the circumference of Earth.
  B.  What do you mean?  Over the poles or 'round the equator?
  C.  Is that 10 km above at all points on the rope, or just one?
  D.  What's the rope made of?  Jute?  Sisal?  Nylon?  How far apart are the supports?  Young's modulus...
  E.  I don't know about that, but I have inherited a rather large quantity of string in 3-inch lengths...

That's a very straightforward math problem.

There is a lot more than the Pythagorean theorem in that question. It would require remember a lot of middle school theorems from a long time ago; frankly calculus would be better.

No. You just need to know circumference = pi times diameter.

Or no, I misunderstood the problem. Either way it's a simple problem that entering math majors should be able to solve.

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