The mass of 4 children, Hilda, Gem, Billy and Gabriel were taken. Only 3 children can be weighed at one time. Hilda, Gem and Billy weighed a total of 79 kg. Gem, Billy and Gabriel weighed a total of 112 kg. Hilda, Billy and Gabriel weighed a total of 49 kg. Hilda, Gem, and Gabriel weighed a total of 120 kg. What is the average mass of the 4 children?
Hilda + Gem + Billy = 79 --- (1)
Gem + Billy + Gabriel = 112 --- (2)
Hilda + Billy + Gabriel = 49 --- (3)
Hilda + Gem + Gabriel = 120 --- (4)

(1) + (2) + (3) + (4)
3(Hilda + Gem + Billy + Gabriel) = 79 + 112 + 49 + 120
3(Hilda + Gem + Billy + Gabriel) = 360
Hilda + Gem + Billy + Gabriel = 360 ÷ 3 = 120

Total mass of the 4 children = 120

Average mass of the 4 children
= 120 ÷ 4
= 30

Answer(s): 30 kg