The mass of 4 children, Shannon, Xuan, Glen and Archie were taken. Only 3 children can be weighed at one time. Shannon, Xuan and Glen weighed a total of 59 kg. Xuan, Glen and Archie weighed a total of 68 kg. Shannon, Glen and Archie weighed a total of 85 kg. Shannon, Xuan, and Archie weighed a total of 136 kg. What is the average mass of the 4 children?
Shannon + Xuan + Glen = 59 --- (1)
Xuan + Glen + Archie = 68 --- (2)
Shannon + Glen + Archie = 85 --- (3)
Shannon + Xuan + Archie = 136 --- (4)

(1) + (2) + (3) + (4)
3(Shannon + Xuan + Glen + Archie) = 59 + 68 + 85 + 136
3(Shannon + Xuan + Glen + Archie) = 348
Shannon + Xuan + Glen + Archie = 348 ÷ 3 = 116

Total mass of the 4 children = 116

Average mass of the 4 children
= 116 ÷ 4
= 29

Answer(s): 29 kg