Say I want to find Pr(A|B). Usually I would just use bayes rule, but some textbooks just assume that A|B works and then just multiply it by P(B). For example in my book, Pr(X>2000)=.4. X and Y are unif distributed from 1000 to 5000. They are independent. Find Pr(X+Y>8000|X>2000). Well they just said, ok look at the region bounded by X+Y>8000 and and X=2000 and Y=1000, Y=5000, X=5000. They found the area of the triangle and divided it by the region formed with X=2000, 5000, Y=1000, 5000. Finally they multiplied it by .4. I don't know what formula they use but obviously it is not Bayes rule.