Calculates the probability of the occurrence of one of two events. If the events are mutually exclusive, then P(A and B)=0 and the general disjunction rule reduces to the restricted one. Usually, A and B are picked to be independent of each other. Thus, P(A and B) = P(A) * P(B).
Consider the probability of getting a heads on either of two tosses of a coin.
Answer: 1/2 + 1/2 - (1/2 * 1/2) = 3/4.