The mass of 4 children, Natalie, Xylia, Bobby and David were taken. Only 3 children can be weighed at one time. Natalie, Xylia and Bobby weighed a total of 47 kg. Xylia, Bobby and David weighed a total of 138 kg. Natalie, Bobby and David weighed a total of 94 kg. Natalie, Xylia, and David weighed a total of 45 kg. What is the average mass of the 4 children?
Natalie + Xylia + Bobby = 47 --- (1)
Xylia + Bobby + David = 138 --- (2)
Natalie + Bobby + David = 94 --- (3)
Natalie + Xylia + David = 45 --- (4)

(1) + (2) + (3) + (4)
3(Natalie + Xylia + Bobby + David) = 47 + 138 + 94 + 45
3(Natalie + Xylia + Bobby + David) = 324
Natalie + Xylia + Bobby + David = 324 ÷ 3 = 108

Total mass of the 4 children = 108

Average mass of the 4 children
= 108 ÷ 4
= 27

Answer(s): 27 kg