The average mass of a group of children is 36kg. If 3/7 of the no. of children are girls and their average mass is 32 kg, find the average mass of the boys.

Answer 1

number of girls = 3x
number of boys = 4x
(check: total is 7x)

total weight of all children = 7x(36) = 252x
total weight of all the girls = 3x(32) = 96x
so the weight of all the boys = 252x-96x = 156x

average weight of boys = 156x/(4x) = 39 kg