[求助]PREP-DS1-147

147.      14800-!-item-!-187;#058&010134

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.

为什么是C?