Bayes’ Rule Let A and B be two events, then P(B|A) =[P(A|B)P(B)]/[ P(A|B) P(B) + P(A|  B) P(  B)] Toss a blue die and a red die: Pr(sum= even| red=2) = Pr(red=2|sum= even)Pr(sum= even ) {Pr(red=2|sum= even)Pr(sum= even )+ Pr(red=2|sum= odd)Pr(sum= odd)}