The 100 members of a club are recorded by age group and sport. Juniors: Swimming 28, Athletics 12. Seniors: Swimming 22, Athletics 38. One member is picked at random from those who take swimming. What is the probability that the member is a junior?

Restrict attention to the swimmers: there are 28 + 22 = 50 of them and 28 are juniors, so the probability is 28/50 = 14/25. Dividing by the whole membership gives 28/100 = 7/25, which ignores the given information, and dividing by the 40 juniors gives 28/40 = 7/10 — the probability that a junior swims, a different question. The value 11/25 = 22/50 is the chance that the swimmer is a senior.