The mass of 4 children, Linda, Risa, David and Justin were taken. Only 3 children can be weighed at one time. Linda, Risa and David weighed a total of 122 kg. Risa, David and Justin weighed a total of 122 kg. Linda, David and Justin weighed a total of 68 kg. Linda, Risa, and Justin weighed a total of 108 kg. What is the average mass of the 4 children?
Linda + Risa + David = 122 --- (1)
Risa + David + Justin = 122 --- (2)
Linda + David + Justin = 68 --- (3)
Linda + Risa + Justin = 108 --- (4)

(1) + (2) + (3) + (4)
3(Linda + Risa + David + Justin) = 122 + 122 + 68 + 108
3(Linda + Risa + David + Justin) = 420
Linda + Risa + David + Justin = 420 ÷ 3 = 140

Total mass of the 4 children = 140

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

Answer(s): 35 kg