GMAT考题：OG2017Q-DS-192

GMAT考题

What is the average (arithmetic mean) of x and y ?

(1) The average of x and 2y is 10.

(2) The average of 2x and 7y is 32.

Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.

Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.

BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.

EACH statement ALONE is sufficient.

Statements (1) and (2) TOGETHER are NOT sufficient.

毕出

题目分析：

求x和y的平均值，即求(x+y)/2，需要求得x和y的值或者x+y的值

（1） x和2y的平均值为10，即(x+2y)/2=10，无法求得x和y或者x+y

（2） 2x和7y的平均值为32，即(2x+7y)/2=32，无法求得x和y或者x+y

结合条件1和条件2，由条件1得x=20-2y，由条件2得2x=64-7y，由这两个联立可得2*(20-2y)=64-7y，解得y=8，代入上式得x=4，所以x和y的均值为(8+4)/2=6。

因此，本题答案为（C），两个条件结合起来才能解题，任何单独一个均不行。

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