The mass of 4 children, Ivory, Abi, Bobby and Tommy were taken. Only 3 children can be weighed at one time. Ivory, Abi and Bobby weighed a total of 104 kg. Abi, Bobby and Tommy weighed a total of 67 kg. Ivory, Bobby and Tommy weighed a total of 45 kg. Ivory, Abi, and Tommy weighed a total of 84 kg. What is the average mass of the 4 children?
Ivory + Abi + Bobby = 104 --- (1)
Abi + Bobby + Tommy = 67 --- (2)
Ivory + Bobby + Tommy = 45 --- (3)
Ivory + Abi + Tommy = 84 --- (4)

(1) + (2) + (3) + (4)
3(Ivory + Abi + Bobby + Tommy) = 104 + 67 + 45 + 84
3(Ivory + Abi + Bobby + Tommy) = 300
Ivory + Abi + Bobby + Tommy = 300 ÷ 3 = 100

Total mass of the 4 children = 100

Average mass of the 4 children
= 100 ÷ 4
= 25

Answer(s): 25 kg