At a school event, the ratio of the number of boys to the number of girls was 3:4. After some boys left, the ratio became 1:2. If there were 120 girls at the school event, how many boys left?

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