Question

Mary 比 Sally 大 $20\%$ ，Sally 比 Danielle 小 $40\%$ 。她们年龄之和是 $23.2$ 岁。Mary 下一个生日时多少岁？

Mary is $20\%$ older than Sally, and Sally is $40\%$ younger than Danielle. The sum of their ages is $23.2$ years. How old will Mary be on her next birthday?

$7$

$8$

$9$

$10$

$11$

答案：B

知识点：百分数一次方程

难度评级：1240

解答：

设 Danielle 为 $x$ 岁。那么 Sally 为 $0.6x$ ，Mary 为 $1.2(0.6x) = 0.72x$ 。

由 $x + 0.6x + 0.72x = 2.32x = 23.2$ ，得 $x = 10$ 。所以 Mary 现在 $0.72(10) = 7.2$ 岁，下一个生日时她将满 $8$ 岁。

因此，正确答案是 B。

Let Danielle be $x$ years old. Then Sally is $0.6x$ and Mary is $1.2(0.6x) = 0.72x.$

The sum $x + 0.6x + 0.72x = 2.32x = 23.2$ gives $x = 10.$ So Mary is $0.72(10) = 7.2$ years old, and on her next birthday she will be $8.$

Thus, the correct answer is B.

2006 AMC 12A 第 7 题

答案：B

难度评级：1240

解答：

其他年份的第 7 题