The mass of 4 children, Irene, Nicole, Sean and Xavier were taken. Only 3 children can be weighed at one time. Irene, Nicole and Sean weighed a total of 128 kg. Nicole, Sean and Xavier weighed a total of 92 kg. Irene, Sean and Xavier weighed a total of 52 kg. Irene, Nicole, and Xavier weighed a total of 100 kg. What is the average mass of the 4 children?
Irene + Nicole + Sean = 128 --- (1)
Nicole + Sean + Xavier = 92 --- (2)
Irene + Sean + Xavier = 52 --- (3)
Irene + Nicole + Xavier = 100 --- (4)

(1) + (2) + (3) + (4)
3(Irene + Nicole + Sean + Xavier) = 128 + 92 + 52 + 100
3(Irene + Nicole + Sean + Xavier) = 372
Irene + Nicole + Sean + Xavier = 372 ÷ 3 = 124

Total mass of the 4 children = 124

Average mass of the 4 children
= 124 ÷ 4
= 31

Answer(s): 31 kg