So female students (f) = 72 + male students (m)
Instead of using f, use m + 72 and add with another m for the number of male students to get:
m + 72 + m =a (for all students).
If 60% are female, then 40% are male. so .40a = m (all males).
Substitute. 40a for all m's above and get:
.40a + 72 + .40a = 1a (For 100%)
.80a + 72 = 1a
72 = .20a
a = 360. (Total number of all students).
Since we want males, multiply .40 by 360 to get 144 males.