请教一道PREP题

smartcoco

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 directors on the task force is $15,000 greater than the average salary of all employees on the task force.

答案是C 即both statement together are sufficient,but neither statement alone is sufficient.

怎么也想不明白，过两天就要考了。希望会的人帮忙解答一下啊！谢谢了！

happypigfy

更正，该题可解

设manager的平均工资为M，director的平均工资为D，employee的平均工资为E，人数分别为m,d,m+d
M+5000=E  D-15000=E
D-M=10000
m*M+d*D=(m+d)*E=(m+d)*(M+5000)
              =mM+5000m+dM+5000d
dD-dM=5000(m+d)
d(D-M)=5000(m+d)
d/(m+d)=5000/(D-M)=5000/10000=1/2  

smartcoco

明白了！谢谢！呵呵