还是问PREP数学，已解决

暗绯莎

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.

答案是C。。。不会做。。。

redo

D=No. of directors, M=No. of managers, Ad=the average salaries of directors, Am=the average salaries of managers, ask for D/(D+M).
Ad-Am=20000.  Ad-15000=(Ad*D+Am*M)/(D+M)=Ad*D/(D+M)+Am*M/(D+M)=Ad*D/(D+M)+(Ad-20000)*(1-D/(D+M)=Ad*D/(D+M)+Ad-Ad*D/(D+M)-20000+20000*D/(D+M)=Ad-20000+20000*D/(D+M).  D/(D+M)=5000/20000=1/4
考试时，碰见这种题，认倒霉吧

暗绯莎

恩，恩，了解了
主要是没有算到最后，想想两个方程四个未知数肯定算不出来的。。。