The mass of 4 children, Barbara, Min, Seth and Carl were taken. Only 3 children can be weighed at one time. Barbara, Min and Seth weighed a total of 55 kg. Min, Seth and Carl weighed a total of 94 kg. Barbara, Seth and Carl weighed a total of 49 kg. Barbara, Min, and Carl weighed a total of 54 kg. What is the average mass of the 4 children?
Barbara + Min + Seth = 55 --- (1)
Min + Seth + Carl = 94 --- (2)
Barbara + Seth + Carl = 49 --- (3)
Barbara + Min + Carl = 54 --- (4)

(1) + (2) + (3) + (4)
3(Barbara + Min + Seth + Carl) = 55 + 94 + 49 + 54
3(Barbara + Min + Seth + Carl) = 252
Barbara + Min + Seth + Carl = 252 ÷ 3 = 84

Total mass of the 4 children = 84

Average mass of the 4 children
= 84 ÷ 4
= 21

Answer(s): 21 kg