The mass of 4 children, Hilda, Natalie, Harry and Zane were taken. Only 3 children can be weighed at one time. Hilda, Natalie and Harry weighed a total of 82 kg. Natalie, Harry and Zane weighed a total of 82 kg. Hilda, Harry and Zane weighed a total of 102 kg. Hilda, Natalie, and Zane weighed a total of 70 kg. What is the average mass of the 4 children?
Hilda + Natalie + Harry = 82 --- (1)
Natalie + Harry + Zane = 82 --- (2)
Hilda + Harry + Zane = 102 --- (3)
Hilda + Natalie + Zane = 70 --- (4)

(1) + (2) + (3) + (4)
3(Hilda + Natalie + Harry + Zane) = 82 + 82 + 102 + 70
3(Hilda + Natalie + Harry + Zane) = 336
Hilda + Natalie + Harry + Zane = 336 ÷ 3 = 112

Total mass of the 4 children = 112

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

Answer(s): 28 kg