单选题 A teacher has two classes. In the first class, there are 14 students. In the second class, there are 22 students. The mean age of the first class is 25. The mean age of the second class is 19. What is the mean age of the two classes put together (rounded to the nearest tenth)?

A、 20.9
B、 21.3
C、 21.7
D、 22.0
E、 22.6

【正确答案】 B

【答案解析】Because the mean age of the 14 students is 25, the sum of all their ages is 14×25=350. Because the mean age of the 22 students is 19, the sum of all their ages is 22×19=418. Thus the sum of all the ages of the 14+22=36 students is 350+418=768, for an overall mean of 768÷36=21.3.