一道概率题 求NN解答！

yaozhixinjie

In a corporation, 50 percent of the male employees and 40 percent of the female employees are at least 35 years old. If 42 percent of all the employees are at least 35 years old, what fraction of the employees in the corporation are females?

A. 3/5

B. 2/3

C. 3/4

D. 4/5

E. 5/6

我是用等量关系列方程，设公司总人数是100.最后算出来male有13人，跟五分之四差的也有点远。不知道这样做对不对。。求NN说一下这道题应该怎么算啊~~

答案是D

谢谢~~

lucifercaters

假设女员工有x人，男员工有y人。
列等式：
0.4x+0.5y= 0.42(x+y)
推出来：0.08y= 0.02x.
也就是女人数量是男人数量4:1的关系。
也就是4/5了。

yaozhixinjie

啊  知道了 设也很重要 求啥设啥