For two events A and B it is given that P(A) = 0.5, P(B) = 0.4 and P(A or B) = 0.9. Which statement is true?

P(A and B) = P(A) + P(B) − P(A or B) = 0.5 + 0.4 − 0.9 = 0, so the two events can never happen together — they are mutually exclusive. Independence would need P(A and B) = P(A) × P(B) = 0.20, which fails. Two events with non-zero probabilities can never be both mutually exclusive and independent.