The mass of 4 children, Zara, Gabby, Paul and Oscar were taken. Only 3 children can be weighed at one time. Zara, Gabby and Paul weighed a total of 67 kg. Gabby, Paul and Oscar weighed a total of 106 kg. Zara, Paul and Oscar weighed a total of 81 kg. Zara, Gabby, and Oscar weighed a total of 58 kg. What is the average mass of the 4 children?
Zara + Gabby + Paul = 67 --- (1)
Gabby + Paul + Oscar = 106 --- (2)
Zara + Paul + Oscar = 81 --- (3)
Zara + Gabby + Oscar = 58 --- (4)

(1) + (2) + (3) + (4)
3(Zara + Gabby + Paul + Oscar) = 67 + 106 + 81 + 58
3(Zara + Gabby + Paul + Oscar) = 312
Zara + Gabby + Paul + Oscar = 312 ÷ 3 = 104

Total mass of the 4 children = 104

Average mass of the 4 children
= 104 ÷ 4
= 26

Answer(s): 26 kg