The mass of 4 children, Cathy, Gwen, Ethan and Howard were taken. Only 3 children can be weighed at one time. Cathy, Gwen and Ethan weighed a total of 57 kg. Gwen, Ethan and Howard weighed a total of 90 kg. Cathy, Ethan and Howard weighed a total of 103 kg. Cathy, Gwen, and Howard weighed a total of 62 kg. What is the average mass of the 4 children?
Cathy + Gwen + Ethan = 57 --- (1)
Gwen + Ethan + Howard = 90 --- (2)
Cathy + Ethan + Howard = 103 --- (3)
Cathy + Gwen + Howard = 62 --- (4)

(1) + (2) + (3) + (4)
3(Cathy + Gwen + Ethan + Howard) = 57 + 90 + 103 + 62
3(Cathy + Gwen + Ethan + Howard) = 312
Cathy + Gwen + Ethan + Howard = 312 ÷ 3 = 104

Total mass of the 4 children = 104

Average mass of the 4 children
= 104 ÷ 4
= 26

Answer(s): 26 kg