The mass of 4 children, Ivory, Rachel, Ben and Justin were taken. Only 3 children can be weighed at one time. Ivory, Rachel and Ben weighed a total of 74 kg. Rachel, Ben and Justin weighed a total of 75 kg. Ivory, Ben and Justin weighed a total of 123 kg. Ivory, Rachel, and Justin weighed a total of 124 kg. What is the average mass of the 4 children?
Ivory + Rachel + Ben = 74 --- (1)
Rachel + Ben + Justin = 75 --- (2)
Ivory + Ben + Justin = 123 --- (3)
Ivory + Rachel + Justin = 124 --- (4)

(1) + (2) + (3) + (4)
3(Ivory + Rachel + Ben + Justin) = 74 + 75 + 123 + 124
3(Ivory + Rachel + Ben + Justin) = 396
Ivory + Rachel + Ben + Justin = 396 ÷ 3 = 132

Total mass of the 4 children = 132

Average mass of the 4 children
= 132 ÷ 4
= 33

Answer(s): 33 kg