3 times as many boys as girls, so write it in the ratio 3:1 (Three boys for every one girl)
Add up all the numbers in the ratio (3 + 1 = 4)
Divide the total number of children (60) by the number you just worked out (4)
Multiply the resulting number (60/4 = 15) by the ratio of girls (1)
15 * 1 = 15, there were 15 girls in the class (And so 45 boys)