∵ Students in a college =100% Let P(M) be the probability who attend Maths class and P(P) the probability who attend Physics classes.
∴P(M)=40%,P(P)=30% ∴ Probability of students who attend only Maths class =P(M)−P(M∩P) =40%−20%=20% Probability of students who attend only Physics class =P(P)−P(M∩P) =30%−20%=10% ∴ Probability that the student chosen attend only one class =20%+10%=30% P=30∕100=3∕10