AMC12 2012 A

AMC12 2012 A · Q4

AMC12 2012 A · Q4. It mainly tests Ratios & proportions.

In a bag of marbles, $\frac{3}{5}$ 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?

一袋弹珠中，$\frac{3}{5}$ 的弹珠是蓝色的，其余是红色的。如果红色弹珠的数量加倍而蓝色弹珠的数量保持不变，那么红色弹珠将占弹珠总数的几分之几？

(A) \frac{2}{5} \frac{2}{5}

(B) \frac{3}{7} \frac{3}{7}

(D) \frac{3}{5} \frac{3}{5}

(E) \frac{4}{5} \frac{4}{5}

Answer

Correct choice: (C)

正确答案：（C）

Solution

Assume that there are 5 total marbles in the bag. The actual number does not matter, since all we care about is the ratios, and the only operation performed on the marbles in the bag is doubling. There are 3 blue marbles in the bag and 2 red marbles. If you double the amount of red marbles, there will still be 3 blue marbles but now there will be 4 red marbles. Thus, the answer is $\boxed{\textbf{(C)}\ \frac{4}{7}}$.

假设袋中共有 5 颗弹珠。实际数量并不重要，因为我们只关心比例，而且对袋中弹珠进行的唯一操作是加倍。袋中有 3 颗蓝色弹珠和 2 颗红色弹珠。如果将红色弹珠数量加倍，蓝色弹珠仍为 3 颗，但红色弹珠变为 4 颗。因此答案是 $\boxed{\textbf{(C)}\ \frac{4}{7}}$。

Topics

AR.004 Ratios & proportions