The mass of 4 children, Xylia, Emma, John and Jeremy were taken. Only 3 children can be weighed at one time. Xylia, Emma and John weighed a total of 143 kg. Emma, John and Jeremy weighed a total of 76 kg. Xylia, John and Jeremy weighed a total of 86 kg. Xylia, Emma, and Jeremy weighed a total of 67 kg. What is the average mass of the 4 children?
Xylia + Emma + John = 143 --- (1)
Emma + John + Jeremy = 76 --- (2)
Xylia + John + Jeremy = 86 --- (3)
Xylia + Emma + Jeremy = 67 --- (4)

(1) + (2) + (3) + (4)
3(Xylia + Emma + John + Jeremy) = 143 + 76 + 86 + 67
3(Xylia + Emma + John + Jeremy) = 372
Xylia + Emma + John + Jeremy = 372 ÷ 3 = 124

Total mass of the 4 children = 124

Average mass of the 4 children
= 124 ÷ 4
= 31

Answer(s): 31 kg