The mass of 4 children, Abi, Jane, David and Jeremy were taken. Only 3 children can be weighed at one time. Abi, Jane and David weighed a total of 104 kg. Jane, David and Jeremy weighed a total of 109 kg. Abi, David and Jeremy weighed a total of 82 kg. Abi, Jane, and Jeremy weighed a total of 113 kg. What is the average mass of the 4 children?
Abi + Jane + David = 104 --- (1)
Jane + David + Jeremy = 109 --- (2)
Abi + David + Jeremy = 82 --- (3)
Abi + Jane + Jeremy = 113 --- (4)

(1) + (2) + (3) + (4)
3(Abi + Jane + David + Jeremy) = 104 + 109 + 82 + 113
3(Abi + Jane + David + Jeremy) = 408
Abi + Jane + David + Jeremy = 408 ÷ 3 = 136

Total mass of the 4 children = 136

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

Answer(s): 34 kg