A community music club has 24 members whose average age is 15 years. Four members aged 20, 19, 17 and 16 leave the club, and four newcomers aged 11, 12, 12 and 13 join it. What is the average age of the club now?

The club had $24\times 15=360$ years of age in it. Out go 20 + 19 + 17 + 16 = 72 years and in come 11 + 12 + 12 + 13 = 48 years, so the total drops by 24 to 336. The club still has 24 members, so the average is 336/24 = 14 years. 15 assumes that four out and four in must leave the average alone, but the group leaving is much older than the group joining. 9 shares the 24-year drop among the 4 who moved rather than among all 24 members. 12 is the newcomers' own average, which ignores the 20 members who never moved.