The mass of 4 children, Shiyun, Lucy, Valen and Ian were taken. Only 3 children can be weighed at one time. Shiyun, Lucy and Valen weighed a total of 61 kg. Lucy, Valen and Ian weighed a total of 128 kg. Shiyun, Valen and Ian weighed a total of 121 kg. Shiyun, Lucy, and Ian weighed a total of 86 kg. What is the average mass of the 4 children?
Shiyun + Lucy + Valen = 61 --- (1)
Lucy + Valen + Ian = 128 --- (2)
Shiyun + Valen + Ian = 121 --- (3)
Shiyun + Lucy + Ian = 86 --- (4)

(1) + (2) + (3) + (4)
3(Shiyun + Lucy + Valen + Ian) = 61 + 128 + 121 + 86
3(Shiyun + Lucy + Valen + Ian) = 396
Shiyun + Lucy + Valen + Ian = 396 ÷ 3 = 132

Total mass of the 4 children = 132

Average mass of the 4 children
= 132 ÷ 4
= 33

Answer(s): 33 kg