The mass of 4 children, Sarah, Irene, Rael and Billy were taken. Only 3 children can be weighed at one time. Sarah, Irene and Rael weighed a total of 87 kg. Irene, Rael and Billy weighed a total of 127 kg. Sarah, Rael and Billy weighed a total of 79 kg. Sarah, Irene, and Billy weighed a total of 79 kg. What is the average mass of the 4 children?
Sarah + Irene + Rael = 87 --- (1)
Irene + Rael + Billy = 127 --- (2)
Sarah + Rael + Billy = 79 --- (3)
Sarah + Irene + Billy = 79 --- (4)

(1) + (2) + (3) + (4)
3(Sarah + Irene + Rael + Billy) = 87 + 127 + 79 + 79
3(Sarah + Irene + Rael + Billy) = 372
Sarah + Irene + Rael + Billy = 372 ÷ 3 = 124

Total mass of the 4 children = 124

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

Answer(s): 31 kg