1. The problem statement, all variables and given/known data I dont undersand why Pr(A l B) = P(AnB)/P(B) 2. Relevant equations conditional probability Pr(A l B) = P(AnB)/P(B) A given B 3. The attempt at a solution I can make a bunch of problems with this definition intuitively, but I really want to understand with sets what is going on behind this equation. Why is that true?. The book is very vague at explaining that.