The mass of 4 children, Kylie, Abi, Jenson and Seth were taken. Only 3 children can be weighed at one time. Kylie, Abi and Jenson weighed a total of 90 kg. Abi, Jenson and Seth weighed a total of 75 kg. Kylie, Jenson and Seth weighed a total of 45 kg. Kylie, Abi, and Seth weighed a total of 102 kg. What is the average mass of the 4 children?
Kylie + Abi + Jenson = 90 --- (1)
Abi + Jenson + Seth = 75 --- (2)
Kylie + Jenson + Seth = 45 --- (3)
Kylie + Abi + Seth = 102 --- (4)

(1) + (2) + (3) + (4)
3(Kylie + Abi + Jenson + Seth) = 90 + 75 + 45 + 102
3(Kylie + Abi + Jenson + Seth) = 312
Kylie + Abi + Jenson + Seth = 312 ÷ 3 = 104

Total mass of the 4 children = 104

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

Answer(s): 26 kg