The mass of 4 children, Tina, Kimberly, Billy and Carl were taken. Only 3 children can be weighed at one time. Tina, Kimberly and Billy weighed a total of 137 kg. Kimberly, Billy and Carl weighed a total of 63 kg. Tina, Billy and Carl weighed a total of 88 kg. Tina, Kimberly, and Carl weighed a total of 120 kg. What is the average mass of the 4 children?
Tina + Kimberly + Billy = 137 --- (1)
Kimberly + Billy + Carl = 63 --- (2)
Tina + Billy + Carl = 88 --- (3)
Tina + Kimberly + Carl = 120 --- (4)

(1) + (2) + (3) + (4)
3(Tina + Kimberly + Billy + Carl) = 137 + 63 + 88 + 120
3(Tina + Kimberly + Billy + Carl) = 408
Tina + Kimberly + Billy + Carl = 408 ÷ 3 = 136

Total mass of the 4 children = 136

Average mass of the 4 children
= 136 ÷ 4
= 34

Answer(s): 34 kg