2007 AMC 10B 第 14 题

Question

一些男生和女生正在洗车，为班级去中国旅行筹款。起初，小组中

40\%

是女生。不久后两名女生离开，两名男生加入，此时小组中

30\%

是女生。起初小组中有多少名女生？

Some boys and girls are having a car wash to raise money for a class trip to China. Initially

40\%

of the group are girls. Shortly thereafter two girls leave and two boys arrive, and then

30\%

of the group are girls. How many girls were initially in the group?

4

6

8

10

12

两人离开、两人加入后，小组总人数不变。

The group size does not change when two leave and two arrive

离开的两名女生代表全组的

40\%-30\%

。

The two departing girls represent

40\%-30\%

of the group

两名女生离开，两名男生加入，所以小组总人数不变。离开的两名女生代表全组的

40\%-30\%=10\%

。

所以小组共有

2\div 0.10=20

人，起初女生人数是

20

人的

40\%

，即

8

。

所以正确答案是 C。

Two girls leave and two boys arrive, so the group size is unchanged. The two girls who left therefore represent

40\%-30\%=10\%

of the group.

Thus the group has

2\div 0.10=20

people, and the original number of girls was

40\%

of

20,

or

8.

Thus, the correct answer is C.

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