The mass of 4 children, Gwen, Yoko, Perry and Andy were taken. Only 3 children can be weighed at one time. Gwen, Yoko and Perry weighed a total of 104 kg. Yoko, Perry and Andy weighed a total of 145 kg. Gwen, Perry and Andy weighed a total of 75 kg. Gwen, Yoko, and Andy weighed a total of 48 kg. What is the average mass of the 4 children?
Gwen + Yoko + Perry = 104 --- (1)
Yoko + Perry + Andy = 145 --- (2)
Gwen + Perry + Andy = 75 --- (3)
Gwen + Yoko + Andy = 48 --- (4)

(1) + (2) + (3) + (4)
3(Gwen + Yoko + Perry + Andy) = 104 + 145 + 75 + 48
3(Gwen + Yoko + Perry + Andy) = 372
Gwen + Yoko + Perry + Andy = 372 ÷ 3 = 124

Total mass of the 4 children = 124

Average mass of the 4 children
= 124 ÷ 4
= 31

Answer(s): 31 kg