The mass of 4 children, Shiyun, Anna, Eric and Howard were taken. Only 3 children can be weighed at one time. Shiyun, Anna and Eric weighed a total of 105 kg. Anna, Eric and Howard weighed a total of 61 kg. Shiyun, Eric and Howard weighed a total of 148 kg. Shiyun, Anna, and Howard weighed a total of 46 kg. What is the average mass of the 4 children?
Shiyun + Anna + Eric = 105 --- (1)
Anna + Eric + Howard = 61 --- (2)
Shiyun + Eric + Howard = 148 --- (3)
Shiyun + Anna + Howard = 46 --- (4)

(1) + (2) + (3) + (4)
3(Shiyun + Anna + Eric + Howard) = 105 + 61 + 148 + 46
3(Shiyun + Anna + Eric + Howard) = 360
Shiyun + Anna + Eric + Howard = 360 ÷ 3 = 120

Total mass of the 4 children = 120

Average mass of the 4 children
= 120 ÷ 4
= 30

Answer(s): 30 kg