The mass of 4 children, Xylia, Abi, Howard and Tommy were taken. Only 3 children can be weighed at one time. Xylia, Abi and Howard weighed a total of 66 kg. Abi, Howard and Tommy weighed a total of 45 kg. Xylia, Howard and Tommy weighed a total of 122 kg. Xylia, Abi, and Tommy weighed a total of 79 kg. What is the average mass of the 4 children?
Xylia + Abi + Howard = 66 --- (1)
Abi + Howard + Tommy = 45 --- (2)
Xylia + Howard + Tommy = 122 --- (3)
Xylia + Abi + Tommy = 79 --- (4)

(1) + (2) + (3) + (4)
3(Xylia + Abi + Howard + Tommy) = 66 + 45 + 122 + 79
3(Xylia + Abi + Howard + Tommy) = 312
Xylia + Abi + Howard + Tommy = 312 ÷ 3 = 104

Total mass of the 4 children = 104

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

Answer(s): 26 kg