Question

一个袋子里有若干弹珠，其中 $\dfrac{3}{5}$ 是蓝色的，其余是红色的。现在把红色弹珠的数量加倍，而蓝色弹珠数量不变。此时红色弹珠占全部弹珠的几分之几？

In a bag of marbles, $\dfrac{3}{5}$ of the marbles are blue and the rest are red. If the number of red marbles is doubled and the number of blue marbles stays the same, what fraction of the marbles will be red?

$\dfrac{2}{5}$

$\dfrac{3}{7}$

$\dfrac{4}{7}$

$\dfrac{3}{5}$

$\dfrac{4}{5}$

答案：C

知识点：比与比例分数

难度评级：870

解答：

不妨设原来共有 $5$ 颗弹珠，其中蓝色有 $3$ 颗，红色有 $2$ 颗。

红色数量加倍后有 $4$ 颗红弹珠，因此红色所占比例为 $\dfrac{4}{4 + 3} = \dfrac{4}{7}.$

所以正确答案是 C。

WLOG, let the number of marbles in the bag be $5.$ Since we only care about ratios, we can do this. Then there are $3$ blue marbles and $2$ red marbles.

Doubling the red marbles gives us $4$ of them. Then the fraction of red marbles is $\dfrac{4}{4 + 3} = \dfrac{4}{7}.$

Thus, C is the correct answer.

2012 AMC 10A 第 7 题

答案：C

难度评级：870

解答：

其他年份的第 7 题