The Addition Rule: Mutually Exclusive Events
P(A or B or C) = P(A) + P(B) + P(C)
This can be represented by the Venn diagram as follows:
If two events are not mutually exclusive the probability of one of them occurring is the sum of the marginal probabilities of the events minus the joint probability of the occurrence of the events.
P(A or B) = P(A) + P(B) - P(A and B)
where A and B are not mutually exclusive events. A venn diagram illustrating the above is