1. The problem statement, all variables and given/known data There are balls numbered 1 through n in a box. Suppose that a boy successively draws a ball from the box, each time replacing the one drawn before drawing another. This continues until the boy draws a ball that he has previously drawn before. Let X denote the number of draws, and compute its probability density function. 2. Relevant equations Probability density function 3. The attempt at a solution I was thinking at first this would simply be 1/n, but I'm unsure how to account for putting the ball back then calculating how many attempts it will take before drawing a previously drawn one.