[求助]请教PREP一道DS题

请教PREP一道DS题:

Each of the students in a certain class received a single grade of P, F, or I. What percent of the students in the class were females?

(1) Of those who received a P, 40 percent are females.

(2) Of those who received either an F or I, 80 percent are males.

这道题答案选E.我选的C.思路：40%P= f, 20%(F + I)= f, so we can get f/(P + F + I), the percent of females in the class.

实在没想明白，马上就要考试了，请大家指点~~

40%P= f, 20%(F + I)= f,

2个f应该是不一样的,因为分别代表2个部分的女性人数,

2部分参照物不一样不存在比例关系

噢,我原本的意思是:40%P=f1, 20%(F + I)= f2, female=f1+f2.结果做题的时候没写清楚，就怎么也找不出原因咯。。。