问一个数学题， prep的。。

coocooper

in a certain senior class, 72% of the male and 80% of the female applied to college, what fraction of the students in senior class are male?
1. there are 840 students in senior class
2. 75% of the students in senior class have applied to college.
答案是B， 请问为什么呀
我对于这种百分比的题目最搞不清楚了
转得头昏。。
还有一周就考了，请大家帮帮忙！！

chinamerica

f: number of the female
m: number of the male

Asked about m / (m+f)

Know:
0.72m+0.8f = 0.75(m+f) i.e., 0.03m = 0.05f or  f= (3/5)m
Then one can compute m / (m+f)= 5/8.  

Condition 2 is enough.

coocooper

嗯，谢谢，我明白了，但是这种类型的题目一般应该怎么想啊，我看到这个类型的题目就头昏。。。。