If two events are independent then if we are given that one is known to have occurred, the probability that the other has or will occur is UNCHANGED! So if we are given that B has occurred (which is what "|B" means), the probability of A is still 3/10. So P(A|B) = P(A) = 3/10. [Caution: don't get "independent events" confused with "mutually exclusive events". Mutually exclusive events cannot both happen. If one happens, the other cannot happen. Independent events CAN both happen, but the probability of either one is not affected if the other one occurs (or doesn't occur).] Edwin