The mass of 4 children, Wendy, Xylia, Cole and Howard were taken. Only 3 children can be weighed at one time. Wendy, Xylia and Cole weighed a total of 133 kg. Xylia, Cole and Howard weighed a total of 80 kg. Wendy, Cole and Howard weighed a total of 81 kg. Wendy, Xylia, and Howard weighed a total of 54 kg. What is the average mass of the 4 children?
Wendy + Xylia + Cole = 133 --- (1)
Xylia + Cole + Howard = 80 --- (2)
Wendy + Cole + Howard = 81 --- (3)
Wendy + Xylia + Howard = 54 --- (4)

(1) + (2) + (3) + (4)
3(Wendy + Xylia + Cole + Howard) = 133 + 80 + 81 + 54
3(Wendy + Xylia + Cole + Howard) = 348
Wendy + Xylia + Cole + Howard = 348 ÷ 3 = 116

Total mass of the 4 children = 116

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

Answer(s): 29 kg