Events A and B are mutually exclusive if A and B cannot both occur; that is, if A ∩ B = Ø. For mutually exclusive events, the probability that either one event or the other occurs is given by the addition law, Pr(A ∩ B) = Pr(A) + Pr(B).

For example, when a die is thrown, the probability of obtaining a ‘one’ is $\frac{1}{6}$, and the probability of obtaining a ‘two’ is $\frac{1}{6}$. These events are mutually exclusive. So the probability of obtaining a ‘one’ or a ‘two’ is equal to $\frac{1}{6}+\frac{1}{6}=\frac{1}{3}$.