The mass of 4 children, Min, Cathy, Luke and Seth were taken. Only 3 children can be weighed at one time. Min, Cathy and Luke weighed a total of 66 kg. Cathy, Luke and Seth weighed a total of 76 kg. Min, Luke and Seth weighed a total of 138 kg. Min, Cathy, and Seth weighed a total of 140 kg. What is the average mass of the 4 children?
Min + Cathy + Luke = 66 --- (1)
Cathy + Luke + Seth = 76 --- (2)
Min + Luke + Seth = 138 --- (3)
Min + Cathy + Seth = 140 --- (4)

(1) + (2) + (3) + (4)
3(Min + Cathy + Luke + Seth) = 66 + 76 + 138 + 140
3(Min + Cathy + Luke + Seth) = 420
Min + Cathy + Luke + Seth = 420 ÷ 3 = 140

Total mass of the 4 children = 140

Average mass of the 4 children
= 140 ÷ 4
= 35

Answer(s): 35 kg