The mass of 4 children, Zoe, Kathy, Eric and Mark were taken. Only 3 children can be weighed at one time. Zoe, Kathy and Eric weighed a total of 63 kg. Kathy, Eric and Mark weighed a total of 70 kg. Zoe, Eric and Mark weighed a total of 116 kg. Zoe, Kathy, and Mark weighed a total of 51 kg. What is the average mass of the 4 children?
Zoe + Kathy + Eric = 63 --- (1)
Kathy + Eric + Mark = 70 --- (2)
Zoe + Eric + Mark = 116 --- (3)
Zoe + Kathy + Mark = 51 --- (4)

(1) + (2) + (3) + (4)
3(Zoe + Kathy + Eric + Mark) = 63 + 70 + 116 + 51
3(Zoe + Kathy + Eric + Mark) = 300
Zoe + Kathy + Eric + Mark = 300 ÷ 3 = 100

Total mass of the 4 children = 100

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

Answer(s): 25 kg