Let, P(A) is the probability for A to fail in an exam and P(B) is the probability for B to fail in an exam. ∵ The probability that either A or B fails is P(A∪B)=P(A)+P(B)−P(A∩B) ⇒P(A∪B)≤P(A)+P(B) ⇒P(A∪B)≤0.2+0.3 {∵P(A)=0.2 and P(B)=0.3 given } ⇒P(A∪B)≤0.5