In a group, 40% are girls. 30% of the girls and 20% of the boys play the piano. A piano player is chosen at random. What is the probability that this player is a girl?

解說

P(girl and piano) = 0.4 × 0.3 = 0.12 and P(boy and piano) = 0.6 × 0.2 = 0.12. Given that the person plays the piano, the probability of being a girl is 0.12 / 0.24 = 0.5.