The mass of 4 children, Nora, Mary, Riordan and Elijah were taken. Only 3 children can be weighed at one time. Nora, Mary and Riordan weighed a total of 62 kg. Mary, Riordan and Elijah weighed a total of 112 kg. Nora, Riordan and Elijah weighed a total of 96 kg. Nora, Mary, and Elijah weighed a total of 150 kg. What is the average mass of the 4 children?
Nora + Mary + Riordan = 62 --- (1)
Mary + Riordan + Elijah = 112 --- (2)
Nora + Riordan + Elijah = 96 --- (3)
Nora + Mary + Elijah = 150 --- (4)

(1) + (2) + (3) + (4)
3(Nora + Mary + Riordan + Elijah) = 62 + 112 + 96 + 150
3(Nora + Mary + Riordan + Elijah) = 420
Nora + Mary + Riordan + Elijah = 420 ÷ 3 = 140

Total mass of the 4 children = 140

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

Answer(s): 35 kg