When two events are mutually exclusive, it means that both events cannot happen at the same time. For instance, getting a six on a die and getting a four on a die. In order to find the probability of one event occurring OR another event occurring (where the events are mutually exclusive), we add the probabilities together.
P(A or B) = P(A) + P(B)
Otherwise, P(A or B) = P(A ⋃ B) = P(A) + P(B) – P(A ⋂ B)