1999 AMC 8 第 13 题
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所有题目均经美国数学协会（MAA）官方合法授权使用。
13.
一个计算机科学营有 名成员，平均年龄为 岁。其中有 个女孩、 个男孩和 个成年人。如果女孩的平均年龄为 岁，男孩的平均年龄为 岁，那么成年人的平均年龄是多少？
The average age of the members of a computer science camp is years. There are girls, boys, and adults. If the average age of the girls is and the average age of the boys is what is the average age of the adults?
答案：C
小提示：
把每个平均年龄转化为年龄总和
Convert each average into a total age
大提示：
从全营总年龄中减去女孩和男孩的总年龄
Subtract the girls’ and boys’ totals from the camp total
解答：
全营所有人的年龄总和为 。
女孩年龄总和为 ，男孩年龄总和为 。
因此成年人的年龄总和为 。
所以成年人的平均年龄为
所以正确答案是 C。
The sum of the ages of everybody at the camp is
The sum of the ages of the girls is and of the boys is
As such, we know that the ages of the adults must be
And therefore, the average age of the adults is then
Thus, C is the correct answer.
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