I have a question on a review that I justcannotcrack.

It goes as follows:

"Event A occurs with probability .4. The conditional probability that A occurs given B occurs is .5, while the probability that A occurs given that B does not occur is .2. What is the conditional Probability that B occurs given A occurs."

So:

P(A) = .4

P(A|B)=.5

P(A|Not B) = .2.

I've tried multiplying things out. I just can't get it .

Can anyone help?