Prove discrete math identity C(n,r)⋅C(r,k) = C(n,k)⋅C(n-k,r-k)?

Prove the identity C(n,r)⋅C(r,k) = C(n,k)⋅C(n-k,r-k), whenever n, r, and k are nonnegative integers with r≤n and k≤r,

a) Using a combinatorial argument
b) using an argument based on the formula for the number of r-combinations of a set with n elements

It really has me stumped!
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