The mass of 4 children, Gabby, Usha, Asher and Justin were taken. Only 3 children can be weighed at one time. Gabby, Usha and Asher weighed a total of 74 kg. Usha, Asher and Justin weighed a total of 49 kg. Gabby, Asher and Justin weighed a total of 71 kg. Gabby, Usha, and Justin weighed a total of 58 kg. What is the average mass of the 4 children?
Gabby + Usha + Asher = 74 --- (1)
Usha + Asher + Justin = 49 --- (2)
Gabby + Asher + Justin = 71 --- (3)
Gabby + Usha + Justin = 58 --- (4)

(1) + (2) + (3) + (4)
3(Gabby + Usha + Asher + Justin) = 74 + 49 + 71 + 58
3(Gabby + Usha + Asher + Justin) = 252
Gabby + Usha + Asher + Justin = 252 ÷ 3 = 84

Total mass of the 4 children = 84

Average mass of the 4 children
= 84 ÷ 4
= 21

Answer(s): 21 kg