AMC8 2010

AMC8 2010 · Q12

AMC8 2010 · Q12. It mainly tests Percent, Arithmetic misc.

Of the 500 balls in a large bag, 80% are red and the rest are blue. How many of the red balls must be removed from the bag so that 75% of the remaining balls are red?

一个大袋子里有500个球，其中80%是红色的，其余是蓝色的。需要移除多少个红球，使得剩下球中75%是红色的？

(A) 25 25

(B) 50 50

(D) 100 100

(E) 150 150

Answer

Correct choice: (D)

正确答案：（D）

Solution

Since 80 percent of the 500 balls are red, there are 400 red balls. Therefore, there must be 100 blue balls. For the 100 blue balls to be 25% or $\dfrac{1}{4}$ of the bag, there must be 400 balls in the bag so 100 red balls must be removed. The answer is $\boxed{\textbf{(D)}\ 100}$.

500个球中有80%是红色的，即400个红球。因此，有100个蓝球。要使100个蓝球占袋子中的25%或$\dfrac{1}{4}$，袋子里必须有400个球，因此需要移除100个红球。答案为$\boxed{\textbf{(D)}\ 100}$。

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