The mass of 4 children, Tina, Kylie, Wesley and Billy were taken. Only 3 children can be weighed at one time. Tina, Kylie and Wesley weighed a total of 84 kg. Kylie, Wesley and Billy weighed a total of 100 kg. Tina, Wesley and Billy weighed a total of 67 kg. Tina, Kylie, and Billy weighed a total of 121 kg. What is the average mass of the 4 children?
Tina + Kylie + Wesley = 84 --- (1)
Kylie + Wesley + Billy = 100 --- (2)
Tina + Wesley + Billy = 67 --- (3)
Tina + Kylie + Billy = 121 --- (4)

(1) + (2) + (3) + (4)
3(Tina + Kylie + Wesley + Billy) = 84 + 100 + 67 + 121
3(Tina + Kylie + Wesley + Billy) = 372
Tina + Kylie + Wesley + Billy = 372 ÷ 3 = 124

Total mass of the 4 children = 124

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

Answer(s): 31 kg