so P(A|B) = P(A intersect B)/ P(B). so, P(A intersect B) is the same as P(A) * P(B) right? so doesn't the P(B) always cancel out, and the answer will always be P(A)? That doesn't makes sense at all.... for example: A family has 2 children, and all possibilites are equally possible. boy boy, boy girl, girl girl, girl boy. what is the probability of getting Both boys, given that at least one is a boy. ========================================== ok, so since each is equally possible, then there is 1/4 chance for each outcome. and to have at least one boy, there is 2 chances, out of the 4 chances, which makes that 1/2. so plug in the formula, and I get the wrong answer....(1/4 * 1/2)/1/4....which is wrong... the book says the answer is 1/3... so how do they do that? I dont understand the formula.