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题库 / Prep2007E1-DS-147
Each employee on a certain task force is either a manager or a director. What percent of the employees on the task force are directors?
(1) The average (arithmetic mean) salary of the managers on the task force is $5,000 less than the average salary of all employees on the task force.
(2) The average (arithmetic mean) salary of the directors on the task force is $15,000 greater than the average salary of all employees on the task force.
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.
技巧：数列里的每个数与平均数的差的和为0.
证明：数列为a1,a2,a3……an，平均数为m，则m=(a1+a2+a3……+an)/n, a1+a2+a3……+an=n*m
a1-m+a2-m+a3-m……+an-m=(a1+a2+a3……+an)-n*m=n*m-n*m=0
本题：设m有x人，d有y人，列方程式：-5000x+15000y=0，解得x=3y，y/(x+y)=1/4
这个方法好！曼哈顿数学里有讲这个 做的时候忘记了。。。
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