Out of the set {1,2,3...100} take any subset of ten numbers and call it T. Prove you can find two disjoint subsets of T such as the sum of the numbers in each subset is the same.
Note: the subsets need not include every number in T; in fact, if you asked for this condition, the problem might be impossible (prove it!).
(In reply to
re(2): Second part by Charlie)
Of course! (Slaps forehead.)