There are 210 students in 7th grade. 90+120=210. You have to cross multiply by setting up two proportions (3/4 and x/120) and multiply 3 by 120 and multiply 4 by x. That leaves you with 360 on one side and 4x on the other. In order to find the value of x you have to isolate it by dividing both sides by 4 to get rid of the 4 on the side that has x. And you do 360/4=90. Then just add the amount of boys with the amount of girls to get your total.