1. The problem statement, all variables and given/known data The probability mass function of a random variable X is: P(X=k) = (r+k-1 C r-1)pr(1-p)k Give an interpretation of X. 2. Relevant equations 3. The attempt at a solution The PMF looks like the setup for a binomial random variable. The first combination looks like you are arranging r-1 successes in r+k-1 slots. And the pr seems like it is giving the probability of r successes occurring. But I don't see what X as a whole is standing for.