I'm assuming you've given a go at it. The best way to tackle this is to just think about what's the probability that both A and B are going to happen? The probability of A and B happening, let me do this in a new color, the probability of A and B happening, I want to stay true to the colors, is equal to, there's a couple of ways you could write this out. 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.