Probability of A happening as well as B = (Conditional Probability of A given B) X (Probability of B happening):

P(A,B)=P(A|B)P(B)

This is called Bayes Theorem.

As P(A,B)=P(B,A), we also have P(A,B)=P(B,A)=P(B|A)P(A)

Combining these two we find:

P(A|B)P(B)=P(B|A)P(A)

This is another form of Bayes Theorem.