The mass of 4 children, Zoe, Linda, Seth and Bryan were taken. Only 3 children can be weighed at one time. Zoe, Linda and Seth weighed a total of 132 kg. Linda, Seth and Bryan weighed a total of 77 kg. Zoe, Seth and Bryan weighed a total of 49 kg. Zoe, Linda, and Bryan weighed a total of 78 kg. What is the average mass of the 4 children?
Zoe + Linda + Seth = 132 --- (1)
Linda + Seth + Bryan = 77 --- (2)
Zoe + Seth + Bryan = 49 --- (3)
Zoe + Linda + Bryan = 78 --- (4)

(1) + (2) + (3) + (4)
3(Zoe + Linda + Seth + Bryan) = 132 + 77 + 49 + 78
3(Zoe + Linda + Seth + Bryan) = 336
Zoe + Linda + Seth + Bryan = 336 ÷ 3 = 112

Total mass of the 4 children = 112

Average mass of the 4 children
= 112 ÷ 4
= 28

Answer(s): 28 kg