Let Y be the number of trials required to observe r successes. List the outcome space for the variable Y and show that the probabilities for Y are giver by the formula P(Y=k)=(k-1)(p^2)(1-p)^(k-2).
Okay I know the outcome space is Y:={2,3,4,...}, the first two trials could be successes or you could never get two successes. The problem looks geometric but I dont undertand the extra (k-1)*p in front.