Q: In a class, 60% like maths (M), 50% like science (S), 30% like both. Find P(M or S). P(M union S) = P(M) + P(S) - P(M intersect S). = 0.6 + 0.5 - 0.3 = 0.8. 80% like at least one. (Without minus 0.3, we would double-count the both-lovers!)