There are 15 tennis balls in a box, of which 9 have NOT previously been used. Three of the balls are randomly chose, played with, and then RETURNED TO THE BOX. Later, another 3 balls are randomly chosen from the box. Find the probability that none of these balls has ever been used. Seems pretty simple, right? Attempted Solution: Event A - 1st round balls have never been used before Event B - 2nd round balls have never been used before We want the probability of events A and B both occuring. Prob(A and B) = Prob(B given A) * Prob(A) Prob(A) = (9C3)/(15C3) = 84/455 Prob(B given A) = (6C3)/(15C3) = 20/455 Prob(A and B) = .008114962 Book says answer is .0893 EDIT: I got it now. I misunderstood the question.