This is equivalent to the probability of A given B times the probability of B. Hopefully that makes sense. The probability that B happens, and that given that B has happened, the probability that A happens. That would also be equal to, obviously this is A and B is happening, or you could do it the other way around. You could do it as the probability that B, the probability that B given A happens, times the probability of A. This also makes sense. What's the probability that A happened? And that given A happened, what's the probability of B?