The mass of 4 children, Fanny, Sabrina, Jack and Rael were taken. Only 3 children can be weighed at one time. Fanny, Sabrina and Jack weighed a total of 111 kg. Sabrina, Jack and Rael weighed a total of 124 kg. Fanny, Jack and Rael weighed a total of 129 kg. Fanny, Sabrina, and Rael weighed a total of 80 kg. What is the average mass of the 4 children?
Fanny + Sabrina + Jack = 111 --- (1)
Sabrina + Jack + Rael = 124 --- (2)
Fanny + Jack + Rael = 129 --- (3)
Fanny + Sabrina + Rael = 80 --- (4)

(1) + (2) + (3) + (4)
3(Fanny + Sabrina + Jack + Rael) = 111 + 124 + 129 + 80
3(Fanny + Sabrina + Jack + Rael) = 444
Fanny + Sabrina + Jack + Rael = 444 ÷ 3 = 148

Total mass of the 4 children = 148

Average mass of the 4 children
= 148 ÷ 4
= 37

Answer(s): 37 kg