The mass of 4 children, Gwen, Lynn, Albert and Billy were taken. Only 3 children can be weighed at one time. Gwen, Lynn and Albert weighed a total of 131 kg. Lynn, Albert and Billy weighed a total of 59 kg. Gwen, Albert and Billy weighed a total of 141 kg. Gwen, Lynn, and Billy weighed a total of 101 kg. What is the average mass of the 4 children?
Gwen + Lynn + Albert = 131 --- (1)
Lynn + Albert + Billy = 59 --- (2)
Gwen + Albert + Billy = 141 --- (3)
Gwen + Lynn + Billy = 101 --- (4)

(1) + (2) + (3) + (4)
3(Gwen + Lynn + Albert + Billy) = 131 + 59 + 141 + 101
3(Gwen + Lynn + Albert + Billy) = 432
Gwen + Lynn + Albert + Billy = 432 ÷ 3 = 144

Total mass of the 4 children = 144

Average mass of the 4 children
= 144 ÷ 4
= 36

Answer(s): 36 kg