The mass of 4 children, Tina, Zara, Charlie and Archie were taken. Only 3 children can be weighed at one time. Tina, Zara and Charlie weighed a total of 126 kg. Zara, Charlie and Archie weighed a total of 82 kg. Tina, Charlie and Archie weighed a total of 116 kg. Tina, Zara, and Archie weighed a total of 108 kg. What is the average mass of the 4 children?
Tina + Zara + Charlie = 126 --- (1)
Zara + Charlie + Archie = 82 --- (2)
Tina + Charlie + Archie = 116 --- (3)
Tina + Zara + Archie = 108 --- (4)

(1) + (2) + (3) + (4)
3(Tina + Zara + Charlie + Archie) = 126 + 82 + 116 + 108
3(Tina + Zara + Charlie + Archie) = 432
Tina + Zara + Charlie + Archie = 432 ÷ 3 = 144

Total mass of the 4 children = 144

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

Answer(s): 36 kg