Question

一个大瓮里有 $100$ 个球，其中 $36\%$ 是红球，其余是蓝球。必须取出多少个蓝球，才能使瓮中红球的百分比变为 $72\%$ ？（不取出红球。）

A large urn contains $100$ balls, of which $36\%$ are red and the rest are blue. How many of the blue balls must be removed so that the percentage of red balls in the urn will be $72\%?$ (No red balls are to be removed.)

$28$

$32$

$36$

$50$

$64$

答案：D

知识点：百分数

难度评级：890

小提示：

红球的数量不变；只有蓝球被取出

The number of red balls never changes; only blue balls leave

大提示：

如果 $36$ 个红球要占全瓮的 $72\%$ ，那么瓮中总数必须是 $36 \div 0.72$ 个

If $36$ red balls are to be $72\%$ of the urn, the urn must hold $36 \div 0.72$ balls

解答：

原来有 $36$ 个红球，而且这个数量保持不变。若红球要占全瓮的 $72\%$ ，瓮中必须有 $36 \div 0.72 = 50$ 个球。因为 $100 - 50 = 50$ ，所以正好要取出 $50$ 个蓝球。

所以正确答案是 D。

There are $36$ red balls, and this count stays fixed. For the red balls to be $72\%$ of the urn, the urn must contain $36 \div 0.72 = 50$ balls. Since $100 - 50 = 50$ , exactly $50$ blue balls are removed.

Thus, the correct answer is D.

2018 AMC 12A 第 1 题

答案：D

难度评级：890

小提示：

大提示：

解答：

其他年份的第 1 题