1. The problem statement, all variables and given/known data Count the solutions in nonnegative integers x1,...,xk to x1 + ... + xk=n 2. Relevant equations There's a Theorem in the chapter (shows that the answer is "n+k-1 choose k-1" but we're not allowed to use it. 3. The attempt at a solution Well, obviously you can just have x1 and make equal to n, or you can make x1,...,xk=n all equal to one, or do a bunch of stuff in between. No stunning revelations for me, though. Ideas for how to construct a formula?