The mass of 4 children, Shiyun, Kathy, Bobby and Howard were taken. Only 3 children can be weighed at one time. Shiyun, Kathy and Bobby weighed a total of 132 kg. Kathy, Bobby and Howard weighed a total of 122 kg. Shiyun, Bobby and Howard weighed a total of 139 kg. Shiyun, Kathy, and Howard weighed a total of 63 kg. What is the average mass of the 4 children?
Shiyun + Kathy + Bobby = 132 --- (1)
Kathy + Bobby + Howard = 122 --- (2)
Shiyun + Bobby + Howard = 139 --- (3)
Shiyun + Kathy + Howard = 63 --- (4)

(1) + (2) + (3) + (4)
3(Shiyun + Kathy + Bobby + Howard) = 132 + 122 + 139 + 63
3(Shiyun + Kathy + Bobby + Howard) = 456
Shiyun + Kathy + Bobby + Howard = 456 ÷ 3 = 152

Total mass of the 4 children = 152

Average mass of the 4 children
= 152 ÷ 4
= 38

Answer(s): 38 kg