Wow, I am surprised #13 is listed as a 1-star problem. I was told a story about this problem by a professor while in class. Supposedly, Edgser Djisktra couldn't sleep one night due to jet lag. He was currently going through a phase in which was exercising the power of thought, practicing thought-exercises such as these without a pencil and paper. While in bed that night, awake due to jet lag, he solved this problem.…
I haven't solved it yet, but it seems to be solvable by sheer diligence? Is there a faster solution? I think in an interview I would ask to write a computer program that solves it. Wait - I guess now I have to try that :-( Edit: which programming language has good support for primes? I thought I saw one recently, but can't find it now.
Re: Classic math puzzles for job interviews
#21Update: OK, now I wrote the program, and in hindsight it was overkill. Just check the candidates for x+y starting from below, and the right solution appears pretty quickly (there are 24 candidates for x+y, but it is not necessary to check them all).