1. The problem statement, all variables and given/known data If (N,R) indicates number of combinations of R objects selected from set of N objects N >=r then prove : R E (N,r) x (M,R-r) = (N+M, R) r=0 E specifies the usual capital sigma notion for a sum. 3. The attempt at a solution Just dont know how to tackle this? Inductive proof may be possible but we have three variables N, M and R, so its non trivial? Any clues?