The mass of 4 children, Fanny, Erika, George and Daniel were taken. Only 3 children can be weighed at one time. Fanny, Erika and George weighed a total of 137 kg. Erika, George and Daniel weighed a total of 110 kg. Fanny, George and Daniel weighed a total of 78 kg. Fanny, Erika, and Daniel weighed a total of 95 kg. What is the average mass of the 4 children?
Fanny + Erika + George = 137 --- (1)
Erika + George + Daniel = 110 --- (2)
Fanny + George + Daniel = 78 --- (3)
Fanny + Erika + Daniel = 95 --- (4)

(1) + (2) + (3) + (4)
3(Fanny + Erika + George + Daniel) = 137 + 110 + 78 + 95
3(Fanny + Erika + George + Daniel) = 420
Fanny + Erika + George + Daniel = 420 ÷ 3 = 140

Total mass of the 4 children = 140

Average mass of the 4 children
= 140 ÷ 4
= 35

Answer(s): 35 kg