The mass of 4 children, Betty, Cathy, Neave and Tommy were taken. Only 3 children can be weighed at one time. Betty, Cathy and Neave weighed a total of 99 kg. Cathy, Neave and Tommy weighed a total of 51 kg. Betty, Neave and Tommy weighed a total of 88 kg. Betty, Cathy, and Tommy weighed a total of 62 kg. What is the average mass of the 4 children?
Betty + Cathy + Neave = 99 --- (1)
Cathy + Neave + Tommy = 51 --- (2)
Betty + Neave + Tommy = 88 --- (3)
Betty + Cathy + Tommy = 62 --- (4)

(1) + (2) + (3) + (4)
3(Betty + Cathy + Neave + Tommy) = 99 + 51 + 88 + 62
3(Betty + Cathy + Neave + Tommy) = 300
Betty + Cathy + Neave + Tommy = 300 ÷ 3 = 100

Total mass of the 4 children = 100

Average mass of the 4 children
= 100 ÷ 4
= 25

Answer(s): 25 kg