The mass of 4 children, Gem, Hilda, Carl and Andy were taken. Only 3 children can be weighed at one time. Gem, Hilda and Carl weighed a total of 106 kg. Hilda, Carl and Andy weighed a total of 129 kg. Gem, Carl and Andy weighed a total of 76 kg. Gem, Hilda, and Andy weighed a total of 121 kg. What is the average mass of the 4 children?
Gem + Hilda + Carl = 106 --- (1)
Hilda + Carl + Andy = 129 --- (2)
Gem + Carl + Andy = 76 --- (3)
Gem + Hilda + Andy = 121 --- (4)

(1) + (2) + (3) + (4)
3(Gem + Hilda + Carl + Andy) = 106 + 129 + 76 + 121
3(Gem + Hilda + Carl + Andy) = 432
Gem + Hilda + Carl + Andy = 432 ÷ 3 = 144

Total mass of the 4 children = 144

Average mass of the 4 children
= 144 ÷ 4
= 36

Answer(s): 36 kg