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?

$4$

$6$

$8$

$10$

$12$

答案：C

知识点：百分数一次方程

难度评级：1330

解答：

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.

2007 AMC 10B 第 14 题

答案：C

难度评级：1330

解答：

其他年份的第 14 题