The mass of 4 children, Irene, Kylie, Julian and Oscar were taken. Only 3 children can be weighed at one time. Irene, Kylie and Julian weighed a total of 120 kg. Kylie, Julian and Oscar weighed a total of 86 kg. Irene, Julian and Oscar weighed a total of 144 kg. Irene, Kylie, and Oscar weighed a total of 130 kg. What is the average mass of the 4 children?
Irene + Kylie + Julian = 120 --- (1)
Kylie + Julian + Oscar = 86 --- (2)
Irene + Julian + Oscar = 144 --- (3)
Irene + Kylie + Oscar = 130 --- (4)

(1) + (2) + (3) + (4)
3(Irene + Kylie + Julian + Oscar) = 120 + 86 + 144 + 130
3(Irene + Kylie + Julian + Oscar) = 480
Irene + Kylie + Julian + Oscar = 480 ÷ 3 = 160

Total mass of the 4 children = 160

Average mass of the 4 children
= 160 ÷ 4
= 40

Answer(s): 40 kg