The mass of 4 children, Cathy, Min, Ethan and Ben were taken. Only 3 children can be weighed at one time. Cathy, Min and Ethan weighed a total of 55 kg. Min, Ethan and Ben weighed a total of 78 kg. Cathy, Ethan and Ben weighed a total of 89 kg. Cathy, Min, and Ben weighed a total of 54 kg. What is the average mass of the 4 children?
Cathy + Min + Ethan = 55 --- (1)
Min + Ethan + Ben = 78 --- (2)
Cathy + Ethan + Ben = 89 --- (3)
Cathy + Min + Ben = 54 --- (4)

(1) + (2) + (3) + (4)
3(Cathy + Min + Ethan + Ben) = 55 + 78 + 89 + 54
3(Cathy + Min + Ethan + Ben) = 276
Cathy + Min + Ethan + Ben = 276 ÷ 3 = 92

Total mass of the 4 children = 92

Average mass of the 4 children
= 92 ÷ 4
= 23

Answer(s): 23 kg