The mass of 4 children, Usha, Joelle, Simon and Jeremy were taken. Only 3 children can be weighed at one time. Usha, Joelle and Simon weighed a total of 115 kg. Joelle, Simon and Jeremy weighed a total of 52 kg. Usha, Simon and Jeremy weighed a total of 63 kg. Usha, Joelle, and Jeremy weighed a total of 142 kg. What is the average mass of the 4 children?
Usha + Joelle + Simon = 115 --- (1)
Joelle + Simon + Jeremy = 52 --- (2)
Usha + Simon + Jeremy = 63 --- (3)
Usha + Joelle + Jeremy = 142 --- (4)

(1) + (2) + (3) + (4)
3(Usha + Joelle + Simon + Jeremy) = 115 + 52 + 63 + 142
3(Usha + Joelle + Simon + Jeremy) = 372
Usha + Joelle + Simon + Jeremy = 372 ÷ 3 = 124

Total mass of the 4 children = 124

Average mass of the 4 children
= 124 ÷ 4
= 31

Answer(s): 31 kg