The mass of 4 children, Elyse, Olivia, Jeremy and Owen were taken. Only 3 children can be weighed at one time. Elyse, Olivia and Jeremy weighed a total of 141 kg. Olivia, Jeremy and Owen weighed a total of 104 kg. Elyse, Jeremy and Owen weighed a total of 90 kg. Elyse, Olivia, and Owen weighed a total of 121 kg. What is the average mass of the 4 children?
Elyse + Olivia + Jeremy = 141 --- (1)
Olivia + Jeremy + Owen = 104 --- (2)
Elyse + Jeremy + Owen = 90 --- (3)
Elyse + Olivia + Owen = 121 --- (4)

(1) + (2) + (3) + (4)
3(Elyse + Olivia + Jeremy + Owen) = 141 + 104 + 90 + 121
3(Elyse + Olivia + Jeremy + Owen) = 456
Elyse + Olivia + Jeremy + Owen = 456 ÷ 3 = 152

Total mass of the 4 children = 152

Average mass of the 4 children
= 152 ÷ 4
= 38

Answer(s): 38 kg