The mass of 4 children, Yoko, Lynn, Dylan and Riordan were taken. Only 3 children can be weighed at one time. Yoko, Lynn and Dylan weighed a total of 111 kg. Lynn, Dylan and Riordan weighed a total of 50 kg. Yoko, Dylan and Riordan weighed a total of 96 kg. Yoko, Lynn, and Riordan weighed a total of 55 kg. What is the average mass of the 4 children?
Yoko + Lynn + Dylan = 111 --- (1)
Lynn + Dylan + Riordan = 50 --- (2)
Yoko + Dylan + Riordan = 96 --- (3)
Yoko + Lynn + Riordan = 55 --- (4)

(1) + (2) + (3) + (4)
3(Yoko + Lynn + Dylan + Riordan) = 111 + 50 + 96 + 55
3(Yoko + Lynn + Dylan + Riordan) = 312
Yoko + Lynn + Dylan + Riordan = 312 ÷ 3 = 104

Total mass of the 4 children = 104

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

Answer(s): 26 kg