A box contains k balls numbered 1,2,...,&. They are drawn one by one without replacement in succession. If X t is the number of the ball drawn at the /th stage, we say that a match occurs if X t = i,...

A box contains k balls numbered 1,2,...,&. They are drawn one by one

without replacement in succession. If X t is the number of the ball drawn

at the /th stage, we say that a match occurs if X t = i, i = 1,2,...,&.

Find the probability of (a) exactly r matches; (b) at least r matches.