Average is defined as: Sum of ages of all the boys/number of boys What is the sum of all the ages? 9 boys at age 10: Total age = 9*10 = 90 15 boys at age 9: Total age = 15*9 = 225 Boys left = 44 - 9 - 15 = 20, who are at age 8 20 boys at age 8: Total age = 20*8 = 160 So total age of the class = 90+225+160 = 475 So the class has 44 boys whose age totals 475. Average age =Hope you got it :)