The mass of 4 children, Fiona, Tammy, Rael and Oscar were taken. Only 3 children can be weighed at one time. Fiona, Tammy and Rael weighed a total of 135 kg. Tammy, Rael and Oscar weighed a total of 146 kg. Fiona, Rael and Oscar weighed a total of 116 kg. Fiona, Tammy, and Oscar weighed a total of 107 kg. What is the average mass of the 4 children?
Fiona + Tammy + Rael = 135 --- (1)
Tammy + Rael + Oscar = 146 --- (2)
Fiona + Rael + Oscar = 116 --- (3)
Fiona + Tammy + Oscar = 107 --- (4)

(1) + (2) + (3) + (4)
3(Fiona + Tammy + Rael + Oscar) = 135 + 146 + 116 + 107
3(Fiona + Tammy + Rael + Oscar) = 504
Fiona + Tammy + Rael + Oscar = 504 ÷ 3 = 168

Total mass of the 4 children = 168

Average mass of the 4 children
= 168 ÷ 4
= 42

Answer(s): 42 kg