you have to minus the probability of intersection of set A and B. the way i understand it: for example, if A is {1,2,3} and B is {3,4,5}, AUB is {1,2,3,4,5}, not {1,2,3,3,4,5}. if you added probability of obtaning A and probability of obtaining B, and did not minus the intersection, you would have calculated the probability of obtaining 3 twice, that's why u have to minus the probability of the intersection ( the probability of obtaining a 3)