What is the addition rule for mutually exclusive events?

• A. P(A ∪ B) = P(A) + P(B)
• B. P(A ∪ B) = P(A)P(B)
• C. P(A ∪ B) = P(A) - P(B)
• D. P(A ∪ B) = max(P(A), P(B))

Answer: (A)

Mutually exclusive events cannot happen at the same time, so there’s no overlap to worry about. When you want the probability that either A or B occurs, you simply add their probabilities: P(A ∪ B) = P(A) + P(B).

This is why the addition rule for mutually exclusive events works: the union counts all outcomes from A and all outcomes from B, with no double-counting because they can’t occur together. If the events could occur together, you’d need to subtract the overlap: P(A ∪ B) = P(A) + P(B) - P(A ∩ B).

The other expressions don’t represent the probability of A or B happening. P(A)P(B) is a product, tied to intersection in some contexts but not the union, and taking the maximum doesn’t reflect the probability of either event occurring. For example, if P(A) = 0.4 and P(B) = 0.3 and A and B are mutually exclusive, P(A ∪ B) = 0.7.