The mass of 4 children, Xandra, Elyse, Liam and Andy were taken. Only 3 children can be weighed at one time. Xandra, Elyse and Liam weighed a total of 132 kg. Elyse, Liam and Andy weighed a total of 54 kg. Xandra, Liam and Andy weighed a total of 126 kg. Xandra, Elyse, and Andy weighed a total of 48 kg. What is the average mass of the 4 children?
Xandra + Elyse + Liam = 132 --- (1)
Elyse + Liam + Andy = 54 --- (2)
Xandra + Liam + Andy = 126 --- (3)
Xandra + Elyse + Andy = 48 --- (4)

(1) + (2) + (3) + (4)
3(Xandra + Elyse + Liam + Andy) = 132 + 54 + 126 + 48
3(Xandra + Elyse + Liam + Andy) = 360
Xandra + Elyse + Liam + Andy = 360 ÷ 3 = 120

Total mass of the 4 children = 120

Average mass of the 4 children
= 120 ÷ 4
= 30

Answer(s): 30 kg