because the number of girl doesn't change .so we use the numbe of girl to get the number of boys.at the begining ,the ratio was 3:4,so thee number of boys was X:120=3:4,so x is 90,it means the number of boy is 90.after soe boys left ,the number of boy is y:120=1:2,so y=60, the number of boys is 60
