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还是问PREP数学,已解决

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楼主
发表于 2010-4-9 22:50:30 | 只看该作者 回帖奖励 |倒序浏览 |阅读模式
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。。。
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沙发
发表于 2010-4-10 14:53:22 | 只看该作者
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
板凳
 楼主| 发表于 2010-4-10 15:52:19 | 只看该作者
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。。。
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