Data Sufficiency

Hello all!
This is my first time posting, I've been (lurking and) learning from all your questions and answers and I see I've got a lot of work to do!
I've got tons of questions, here's my first, can someone please explain the answer to this?

In a certain senior class, 72 percent of the male students and 80 percent of the female students have applied to college. What fraction of the students in the senior class are male?

(1) There are 840 students in the senior class
(2) 75 percent of the students in the senior class have applied to college.


Here's what I did:

T(total students) =m+f
C = .75m+ .8f

1) T = 840 NS
2) .75T = .75m + .8f

1&2) 630 = .75m + .8f


Also, can someone tell me how to embed the image/attachment in my post?

Thanks!