1. The problem statement, all variables and given/known data Ten cards are face down in a row on a table. Exactly one of them is an ace. You turn the cards over oen at a time, moving from left to right. Let X be the random variable for the number of cards turned before the ace is turned over. What is the probability function for X? 2. Relevant equations P(a|b)=P(a&b) / P(B) 3. The attempt at a solution P(X=0) = P(1st card is the ace)=1/10 P(X=1)=P(2nd card is the ace|1st card is not ace) * P(1st card is not ace)=9/10 * 1/9 P(X=2)=9/10 * 8/9 * 1/8 = 1/10 so p(x)=1/10 for x=0,1,...,9 I am having trouble understanding how the book arrived at the solution. For P(X=1), it appears to me that they manipulated the equation P(a|b)=P(a&b) / P(B) to be P(a|b) * P(b) = P(a&b). So they are solving for P(a&b). But isn't the key to solve for P(a|b)?