1. The problem statement, all variables and given/known data Suppose n is a natural number and A is a subset of natural number with n elements. Prove that a subset of A exists that the sum of its elements is dividable by n. 3. The attempt at a solution well, This problem is harder than I can solve it. I first tried to use the pigeonhole principle to prove the existence of such a subset of A. well, It wasn't easy to prove it that way as I thought. so, now I feel that I'm stuck and have no idea of how to solve it. Any ideas?