The mass of 4 children, Nora, Marion, Archie and Sam were taken. Only 3 children can be weighed at one time. Nora, Marion and Archie weighed a total of 82 kg. Marion, Archie and Sam weighed a total of 108 kg. Nora, Archie and Sam weighed a total of 119 kg. Nora, Marion, and Sam weighed a total of 111 kg. What is the average mass of the 4 children?
Nora + Marion + Archie = 82 --- (1)
Marion + Archie + Sam = 108 --- (2)
Nora + Archie + Sam = 119 --- (3)
Nora + Marion + Sam = 111 --- (4)

(1) + (2) + (3) + (4)
3(Nora + Marion + Archie + Sam) = 82 + 108 + 119 + 111
3(Nora + Marion + Archie + Sam) = 420
Nora + Marion + Archie + Sam = 420 ÷ 3 = 140

Total mass of the 4 children = 140

Average mass of the 4 children
= 140 ÷ 4
= 35

Answer(s): 35 kg