The mass of 4 children, Xylia, Eva, Henry and Japheth were taken. Only 3 children can be weighed at one time. Xylia, Eva and Henry weighed a total of 96 kg. Eva, Henry and Japheth weighed a total of 106 kg. Xylia, Henry and Japheth weighed a total of 77 kg. Xylia, Eva, and Japheth weighed a total of 129 kg. What is the average mass of the 4 children?
Xylia + Eva + Henry = 96 --- (1)
Eva + Henry + Japheth = 106 --- (2)
Xylia + Henry + Japheth = 77 --- (3)
Xylia + Eva + Japheth = 129 --- (4)

(1) + (2) + (3) + (4)
3(Xylia + Eva + Henry + Japheth) = 96 + 106 + 77 + 129
3(Xylia + Eva + Henry + Japheth) = 408
Xylia + Eva + Henry + Japheth = 408 ÷ 3 = 136

Total mass of the 4 children = 136

Average mass of the 4 children
= 136 ÷ 4
= 34

Answer(s): 34 kg