SOLUTION: out of the 44 boys in a class 9 are of the age of 10, 15 at the age of 9, and the rest are at the age of 8. Find their average age

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 :)