The mass of 4 children, Risa, Nora, Brandon and Daniel were taken. Only 3 children can be weighed at one time. Risa, Nora and Brandon weighed a total of 98 kg. Nora, Brandon and Daniel weighed a total of 72 kg. Risa, Brandon and Daniel weighed a total of 75 kg. Risa, Nora, and Daniel weighed a total of 67 kg. What is the average mass of the 4 children?
Risa + Nora + Brandon = 98 --- (1)
Nora + Brandon + Daniel = 72 --- (2)
Risa + Brandon + Daniel = 75 --- (3)
Risa + Nora + Daniel = 67 --- (4)

(1) + (2) + (3) + (4)
3(Risa + Nora + Brandon + Daniel) = 98 + 72 + 75 + 67
3(Risa + Nora + Brandon + Daniel) = 312
Risa + Nora + Brandon + Daniel = 312 ÷ 3 = 104

Total mass of the 4 children = 104

Average mass of the 4 children
= 104 ÷ 4
= 26

Answer(s): 26 kg