http://projecteuler.net/

Project Euler is a series of math/computation challenges, stuff like "find the sum of all multiples of three or five below one thousand", "what's the 10001st prime?", and "Determine the sum of all integers which are the sum of exactly one of the 50-element subsets of S, i.e. find sum(U(S,50))".

There's a little more to that last one, and it's puzzle 201 out of (currently) 202 puzzles, so it's one of the trickier ones.

Many of the puzzles are appropriate for high schoolers who know a little algebra and a little computer programming. Several require some knowledge of algorithms to be computed in reasonable lengths of time.

Thanks to XKCD for wasting even MORE of my time.