The mass of 4 children, Jane, Sarah, Ryan and Archie were taken. Only 3 children can be weighed at one time. Jane, Sarah and Ryan weighed a total of 142 kg. Sarah, Ryan and Archie weighed a total of 67 kg. Jane, Ryan and Archie weighed a total of 53 kg. Jane, Sarah, and Archie weighed a total of 98 kg. What is the average mass of the 4 children?
Jane + Sarah + Ryan = 142 --- (1)
Sarah + Ryan + Archie = 67 --- (2)
Jane + Ryan + Archie = 53 --- (3)
Jane + Sarah + Archie = 98 --- (4)

(1) + (2) + (3) + (4)
3(Jane + Sarah + Ryan + Archie) = 142 + 67 + 53 + 98
3(Jane + Sarah + Ryan + Archie) = 360
Jane + Sarah + Ryan + Archie = 360 ÷ 3 = 120

Total mass of the 4 children = 120

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

Answer(s): 30 kg