The mass of 4 children, Rachel, Julie, Bryan and Seth were taken. Only 3 children can be weighed at one time. Rachel, Julie and Bryan weighed a total of 118 kg. Julie, Bryan and Seth weighed a total of 72 kg. Rachel, Bryan and Seth weighed a total of 70 kg. Rachel, Julie, and Seth weighed a total of 64 kg. What is the average mass of the 4 children?
Rachel + Julie + Bryan = 118 --- (1)
Julie + Bryan + Seth = 72 --- (2)
Rachel + Bryan + Seth = 70 --- (3)
Rachel + Julie + Seth = 64 --- (4)

(1) + (2) + (3) + (4)
3(Rachel + Julie + Bryan + Seth) = 118 + 72 + 70 + 64
3(Rachel + Julie + Bryan + Seth) = 324
Rachel + Julie + Bryan + Seth = 324 ÷ 3 = 108

Total mass of the 4 children = 108

Average mass of the 4 children
= 108 ÷ 4
= 27

Answer(s): 27 kg