The mass of 4 children, Nora, Olivia, Archie and Lee were taken. Only 3 children can be weighed at one time. Nora, Olivia and Archie weighed a total of 50 kg. Olivia, Archie and Lee weighed a total of 49 kg. Nora, Archie and Lee weighed a total of 107 kg. Nora, Olivia, and Lee weighed a total of 94 kg. What is the average mass of the 4 children?
Nora + Olivia + Archie = 50 --- (1)
Olivia + Archie + Lee = 49 --- (2)
Nora + Archie + Lee = 107 --- (3)
Nora + Olivia + Lee = 94 --- (4)

(1) + (2) + (3) + (4)
3(Nora + Olivia + Archie + Lee) = 50 + 49 + 107 + 94
3(Nora + Olivia + Archie + Lee) = 300
Nora + Olivia + Archie + Lee = 300 ÷ 3 = 100

Total mass of the 4 children = 100

Average mass of the 4 children
= 100 ÷ 4
= 25

Answer(s): 25 kg