0 and then the main thing here is essentially how you kind of keep track of probability the probability of the union of two possible outcomes a and B is a probability of a plus probability of B but it's possible that a and B happen at the same time and then you're double counting down as you subtract back out the probability that a and B happened at the same time which is a intersection B and that gives you the probability of the Union pictorially looks like this you have your Omega space which has all possible events in it then there is