The mass of 4 children, Zoe, Nora, Daniel and Ian were taken. Only 3 children can be weighed at one time. Zoe, Nora and Daniel weighed a total of 88 kg. Nora, Daniel and Ian weighed a total of 66 kg. Zoe, Daniel and Ian weighed a total of 139 kg. Zoe, Nora, and Ian weighed a total of 67 kg. What is the average mass of the 4 children?
Zoe + Nora + Daniel = 88 --- (1)
Nora + Daniel + Ian = 66 --- (2)
Zoe + Daniel + Ian = 139 --- (3)
Zoe + Nora + Ian = 67 --- (4)

(1) + (2) + (3) + (4)
3(Zoe + Nora + Daniel + Ian) = 88 + 66 + 139 + 67
3(Zoe + Nora + Daniel + Ian) = 360
Zoe + Nora + Daniel + Ian = 360 ÷ 3 = 120

Total mass of the 4 children = 120

Average mass of the 4 children
= 120 ÷ 4
= 30

Answer(s): 30 kg