The mass of 4 children, Sabrina, Rachel, Julian and Ken were taken. Only 3 children can be weighed at one time. Sabrina, Rachel and Julian weighed a total of 70 kg. Rachel, Julian and Ken weighed a total of 69 kg. Sabrina, Julian and Ken weighed a total of 112 kg. Sabrina, Rachel, and Ken weighed a total of 121 kg. What is the average mass of the 4 children?
Sabrina + Rachel + Julian = 70 --- (1)
Rachel + Julian + Ken = 69 --- (2)
Sabrina + Julian + Ken = 112 --- (3)
Sabrina + Rachel + Ken = 121 --- (4)

(1) + (2) + (3) + (4)
3(Sabrina + Rachel + Julian + Ken) = 70 + 69 + 112 + 121
3(Sabrina + Rachel + Julian + Ken) = 372
Sabrina + Rachel + Julian + Ken = 372 ÷ 3 = 124

Total mass of the 4 children = 124

Average mass of the 4 children
= 124 ÷ 4
= 31

Answer(s): 31 kg