The mass of 4 children, Cathy, Gwen, Owen and Archie were taken. Only 3 children can be weighed at one time. Cathy, Gwen and Owen weighed a total of 126 kg. Gwen, Owen and Archie weighed a total of 110 kg. Cathy, Owen and Archie weighed a total of 61 kg. Cathy, Gwen, and Archie weighed a total of 111 kg. What is the average mass of the 4 children?
Cathy + Gwen + Owen = 126 --- (1)
Gwen + Owen + Archie = 110 --- (2)
Cathy + Owen + Archie = 61 --- (3)
Cathy + Gwen + Archie = 111 --- (4)

(1) + (2) + (3) + (4)
3(Cathy + Gwen + Owen + Archie) = 126 + 110 + 61 + 111
3(Cathy + Gwen + Owen + Archie) = 408
Cathy + Gwen + Owen + Archie = 408 ÷ 3 = 136

Total mass of the 4 children = 136

Average mass of the 4 children
= 136 ÷ 4
= 34

Answer(s): 34 kg