The mass of 4 children, Wendy, Zara, Will and Seth were taken. Only 3 children can be weighed at one time. Wendy, Zara and Will weighed a total of 149 kg. Zara, Will and Seth weighed a total of 81 kg. Wendy, Will and Seth weighed a total of 84 kg. Wendy, Zara, and Seth weighed a total of 142 kg. What is the average mass of the 4 children?
Wendy + Zara + Will = 149 --- (1)
Zara + Will + Seth = 81 --- (2)
Wendy + Will + Seth = 84 --- (3)
Wendy + Zara + Seth = 142 --- (4)

(1) + (2) + (3) + (4)
3(Wendy + Zara + Will + Seth) = 149 + 81 + 84 + 142
3(Wendy + Zara + Will + Seth) = 456
Wendy + Zara + Will + Seth = 456 ÷ 3 = 152

Total mass of the 4 children = 152

Average mass of the 4 children
= 152 ÷ 4
= 38

Answer(s): 38 kg