IMO C

1) talks about the total number of students in the class.
It doesn't lead to any concrete solution.

2) gives the % of students as a whole.
Assuming the total number of male students to be x and total number of students to be y, we can arrive at the following equation:

72% of x + 80% of (y-x) = 75% of y
This doesn't provide a concrete solution either.

Combining both, we have

72% of x + 80% of (840-x) = 75% of 840

On solving this we get x = 525.

Hence % of males in the class is 525/840*100.

Hence IMO C

but the answer is B according to GMAT prep

Sorry mate,

My bad. Just realized my mistake.

Assume men = x and women = y.

Hence,
72% of x + 80% of y = 75 % of (x+y)

=> 72x + 80y = 75x + 75y

=> x/y = 5/3

=> x/(x+y) = 5/8.

Hence, B is the answer.

And apologies for giving the wrong explanation earlier