The mass of 4 children, Shiyun, Erika, Harry and Jenson were taken. Only 3 children can be weighed at one time. Shiyun, Erika and Harry weighed a total of 139 kg. Erika, Harry and Jenson weighed a total of 107 kg. Shiyun, Harry and Jenson weighed a total of 68 kg. Shiyun, Erika, and Jenson weighed a total of 130 kg. What is the average mass of the 4 children?
Shiyun + Erika + Harry = 139 --- (1)
Erika + Harry + Jenson = 107 --- (2)
Shiyun + Harry + Jenson = 68 --- (3)
Shiyun + Erika + Jenson = 130 --- (4)

(1) + (2) + (3) + (4)
3(Shiyun + Erika + Harry + Jenson) = 139 + 107 + 68 + 130
3(Shiyun + Erika + Harry + Jenson) = 444
Shiyun + Erika + Harry + Jenson = 444 ÷ 3 = 148

Total mass of the 4 children = 148

Average mass of the 4 children
= 148 ÷ 4
= 37

Answer(s): 37 kg