The mass of 4 children, Yoko, Hilda, Riordan and Tommy were taken. Only 3 children can be weighed at one time. Yoko, Hilda and Riordan weighed a total of 98 kg. Hilda, Riordan and Tommy weighed a total of 86 kg. Yoko, Riordan and Tommy weighed a total of 80 kg. Yoko, Hilda, and Tommy weighed a total of 84 kg. What is the average mass of the 4 children?
Yoko + Hilda + Riordan = 98 --- (1)
Hilda + Riordan + Tommy = 86 --- (2)
Yoko + Riordan + Tommy = 80 --- (3)
Yoko + Hilda + Tommy = 84 --- (4)

(1) + (2) + (3) + (4)
3(Yoko + Hilda + Riordan + Tommy) = 98 + 86 + 80 + 84
3(Yoko + Hilda + Riordan + Tommy) = 348
Yoko + Hilda + Riordan + Tommy = 348 ÷ 3 = 116

Total mass of the 4 children = 116

Average mass of the 4 children
= 116 ÷ 4
= 29

Answer(s): 29 kg