The mass of 4 children, Xuan, Zara, Sean and Eric were taken. Only 3 children can be weighed at one time. Xuan, Zara and Sean weighed a total of 87 kg. Zara, Sean and Eric weighed a total of 114 kg. Xuan, Sean and Eric weighed a total of 100 kg. Xuan, Zara, and Eric weighed a total of 59 kg. What is the average mass of the 4 children?
Xuan + Zara + Sean = 87 --- (1)
Zara + Sean + Eric = 114 --- (2)
Xuan + Sean + Eric = 100 --- (3)
Xuan + Zara + Eric = 59 --- (4)

(1) + (2) + (3) + (4)
3(Xuan + Zara + Sean + Eric) = 87 + 114 + 100 + 59
3(Xuan + Zara + Sean + Eric) = 360
Xuan + Zara + Sean + Eric = 360 ÷ 3 = 120

Total mass of the 4 children = 120

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

Answer(s): 30 kg