Hi,
There are 840 students in the senior class
75 percent of the students in the senior class have applied to college
Let x represent the number of females. then (840 -x ) represents the number of males.
question states
.72(840-x) + .80x = .75*840
solving for x
.08x = .03*840
x = 315 of the senior class are females
840-315 = 525 are males
checking our answer
.72*525 + .80*315 = 378 + 252 = .75*840 = 630