The mass of 4 children, Gabby, Dana, Rael and Riordan were taken. Only 3 children can be weighed at one time. Gabby, Dana and Rael weighed a total of 107 kg. Dana, Rael and Riordan weighed a total of 48 kg. Gabby, Rael and Riordan weighed a total of 65 kg. Gabby, Dana, and Riordan weighed a total of 140 kg. What is the average mass of the 4 children?
Gabby + Dana + Rael = 107 --- (1)
Dana + Rael + Riordan = 48 --- (2)
Gabby + Rael + Riordan = 65 --- (3)
Gabby + Dana + Riordan = 140 --- (4)

(1) + (2) + (3) + (4)
3(Gabby + Dana + Rael + Riordan) = 107 + 48 + 65 + 140
3(Gabby + Dana + Rael + Riordan) = 360
Gabby + Dana + Rael + Riordan = 360 ÷ 3 = 120

Total mass of the 4 children = 120

Average mass of the 4 children
= 120 ÷ 4
= 30

Answer(s): 30 kg