The mass of 4 children, Elyse, Usha, Billy and Julian were taken. Only 3 children can be weighed at one time. Elyse, Usha and Billy weighed a total of 56 kg. Usha, Billy and Julian weighed a total of 46 kg. Elyse, Billy and Julian weighed a total of 68 kg. Elyse, Usha, and Julian weighed a total of 46 kg. What is the average mass of the 4 children?
Elyse + Usha + Billy = 56 --- (1)
Usha + Billy + Julian = 46 --- (2)
Elyse + Billy + Julian = 68 --- (3)
Elyse + Usha + Julian = 46 --- (4)

(1) + (2) + (3) + (4)
3(Elyse + Usha + Billy + Julian) = 56 + 46 + 68 + 46
3(Elyse + Usha + Billy + Julian) = 216
Elyse + Usha + Billy + Julian = 216 ÷ 3 = 72

Total mass of the 4 children = 72

Average mass of the 4 children
= 72 ÷ 4
= 18

Answer(s): 18 kg