Final answer:
When two events are mutually exclusive, the probability that either one occurs is the sum of their individual probabilities, because both cannot occur simultaneously.
Step-by-step explanation:
If two events A and B are mutually exclusive, the special rule of addition, also known as the sum rule, states that the probability (P) that at least one of the events occurs (A or B) is equal to the sum of their individual probabilities. Mathematically, this is expressed as P(A OR B) = P(A) + P(B). This is because the probability of A and B occurring together (P(A AND B)) is zero, reflecting the fact that mutually exclusive events cannot occur at the same time.
For example, if we have a deck of cards and event A represents drawing a red card and event B represents drawing a black card, A and B are mutually exclusive since no card can be both red and black at the same time. Therefore, the probability of drawing a card that is either red or black would be P(A OR B) = P(A) + P(B).