GMAT
题目
A cookery course is divided into two groups. In group X, the average score in the test was 76. In group Y, the average score in the test was 70. If the average score of all 30 trainees in the course was 74, how many trainees are in group X?
解析
x 的平均值是 76
y 的平均值是 70
两者的平均值（x+y=30）是 74
76x+70y/(x+y) =74
76x+70y=74*30
76x+70y=2220
x+y=30
y=30-x
76x+70(30-x)=2220
76x+2100-70x=2220
76x-70x=2220-2100
6x=120
x=20
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