请教一道Prep Data Sufficiency的问题

Tiff323

Each Employee on a task force is either a manager or a director. What percent of the employees on the task force are directors?
(1) The average 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 salary of the directors on the task force is $15,000 greater than the average salary of all employees on the task force

自己分析也能想到答案，但是不知道如何用算式表达出来。麻烦各位朋友讲解一下思路。谢谢了
答案 ：
C

Tiff323

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