The mass of 4 children, Fanny, Gabby, Luis and Albert were taken. Only 3 children can be weighed at one time. Fanny, Gabby and Luis weighed a total of 118 kg. Gabby, Luis and Albert weighed a total of 138 kg. Fanny, Luis and Albert weighed a total of 96 kg. Fanny, Gabby, and Albert weighed a total of 128 kg. What is the average mass of the 4 children?
Fanny + Gabby + Luis = 118 --- (1)
Gabby + Luis + Albert = 138 --- (2)
Fanny + Luis + Albert = 96 --- (3)
Fanny + Gabby + Albert = 128 --- (4)

(1) + (2) + (3) + (4)
3(Fanny + Gabby + Luis + Albert) = 118 + 138 + 96 + 128
3(Fanny + Gabby + Luis + Albert) = 480
Fanny + Gabby + Luis + Albert = 480 ÷ 3 = 160

Total mass of the 4 children = 160

Average mass of the 4 children
= 160 ÷ 4
= 40

Answer(s): 40 kg