The mass of 4 children, Ivory, Xylia, John and Nick were taken. Only 3 children can be weighed at one time. Ivory, Xylia and John weighed a total of 53 kg. Xylia, John and Nick weighed a total of 123 kg. Ivory, John and Nick weighed a total of 62 kg. Ivory, Xylia, and Nick weighed a total of 122 kg. What is the average mass of the 4 children?
Ivory + Xylia + John = 53 --- (1)
Xylia + John + Nick = 123 --- (2)
Ivory + John + Nick = 62 --- (3)
Ivory + Xylia + Nick = 122 --- (4)

(1) + (2) + (3) + (4)
3(Ivory + Xylia + John + Nick) = 53 + 123 + 62 + 122
3(Ivory + Xylia + John + Nick) = 360
Ivory + Xylia + John + Nick = 360 ÷ 3 = 120

Total mass of the 4 children = 120

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

Answer(s): 30 kg