I need help answering the following question on hash colliisions. I do not think my answers are correct.

Suppose that h is a secure hash that generates an n-bit hash value. a) What is the expected number of hashes that must be computed to find one collision? 2^n/2-1 b) What is the expected number of hashes that must be computed to find 10 collisions? That is, what is the expected number of hashes that must be computed to find pairs (Xi, Zi) with h(xi) = h(zi), for i = 0, 1, 2, ..., 9? c) What is the expected number of hashes that must be computed to find m collisions? 2^n/2-m