The mass of 4 children, Gillian, Ivory, Riordan and Nick were taken. Only 3 children can be weighed at one time. Gillian, Ivory and Riordan weighed a total of 110 kg. Ivory, Riordan and Nick weighed a total of 132 kg. Gillian, Riordan and Nick weighed a total of 86 kg. Gillian, Ivory, and Nick weighed a total of 140 kg. What is the average mass of the 4 children?
Gillian + Ivory + Riordan = 110 --- (1)
Ivory + Riordan + Nick = 132 --- (2)
Gillian + Riordan + Nick = 86 --- (3)
Gillian + Ivory + Nick = 140 --- (4)

(1) + (2) + (3) + (4)
3(Gillian + Ivory + Riordan + Nick) = 110 + 132 + 86 + 140
3(Gillian + Ivory + Riordan + Nick) = 468
Gillian + Ivory + Riordan + Nick = 468 ÷ 3 = 156

Total mass of the 4 children = 156

Average mass of the 4 children
= 156 ÷ 4
= 39

Answer(s): 39 kg