2018 AMC 12A 第 1 题

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

知识点：百分数

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

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

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.

其他年份的第 1 题