AMC8 1999

AMC8 1999 · Q13

AMC8 1999 · Q13. It mainly tests Averages (mean).

The average age of the 40 members of a computer science camp is 17 years. There are 20 girls, 15 boys, and 5 adults. If the average age of the girls is 15 and the average age of the boys is 16, what is the average age of the adults?

一个计算机科学夏令营有40名成员，平均年龄17岁。其中20名女孩、15名男孩和5名成人。女孩平均年龄15岁，男孩平均年龄16岁，成人平均年龄是多少？

(A) 26 26

(B) 27 27

(D) 29 29

(E) 30 30

Answer

Correct choice: (C)

正确答案：（C）

Solution

First, find the total amount of the girl's ages and add it to the total amount of the boy's ages. It equals $(20)(15)+(15)(16)=540$. The total amount of everyone's ages can be found from the average age, $17\cdot40=680$. Then you do $680-540=140$ to find the sum of the adult's ages. The average age of an adult is divided among the five of them, $140\div5=\boxed{\text{(C)}\ 28}$.

先求女孩总年龄和男孩总年龄之和：$(20)(15)+(15)(16)=540$。全体总年龄为平均年龄乘总数：$17\cdot40=680$。成人总年龄为$680-540=140$。5名成人的平均年龄为$140\div5=\boxed{\text{(C)}\ 28}$。

Topics

AR.006 Averages (mean)