Question

一个计算机科学营有 $40$ 名成员，平均年龄为 $17$ 岁。其中有 $20$ 个女孩、 $15$ 个男孩和 $5$ 个成年人。如果女孩的平均年龄为 $15$ 岁，男孩的平均年龄为 $16$ 岁，那么成年人的平均年龄是多少？

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?

$26$

$27$

$28$

$29$

$30$

答案：C

知识点：平均数

难度评级：1150

解答：

全营所有人的年龄总和为 $40 \cdot 17 = 680$ 。

女孩年龄总和为 $20 \cdot 15 = 300$ ，男孩年龄总和为 $15 \cdot 16 = 240$ 。

因此成年人的年龄总和为 $680 - 300 - 240 = 140$ 。

所以成年人的平均年龄为 $140 \div 5 = 28.$

所以正确答案是 C。

The sum of the ages of everybody at the camp is $40 \cdot 17 = 680.$

The sum of the ages of the girls is $20 \cdot 15 = 300$ and of the boys is $15 \cdot 16 = 240.$

As such, we know that the ages of the adults must be $680 - 300 - 240 = 140.$

And therefore, the average age of the adults is then $140 \div 5 = 28.$

Thus, C is the correct answer.

1999 AMC 8 第 13 题

答案：C

难度评级：1150

解答：

其他年份的第 13 题