The mass of 4 children, Kylie, Tammy, Carl and Reggie were taken. Only 3 children can be weighed at one time. Kylie, Tammy and Carl weighed a total of 129 kg. Tammy, Carl and Reggie weighed a total of 112 kg. Kylie, Carl and Reggie weighed a total of 56 kg. Kylie, Tammy, and Reggie weighed a total of 63 kg. What is the average mass of the 4 children?
Kylie + Tammy + Carl = 129 --- (1)
Tammy + Carl + Reggie = 112 --- (2)
Kylie + Carl + Reggie = 56 --- (3)
Kylie + Tammy + Reggie = 63 --- (4)

(1) + (2) + (3) + (4)
3(Kylie + Tammy + Carl + Reggie) = 129 + 112 + 56 + 63
3(Kylie + Tammy + Carl + Reggie) = 360
Kylie + Tammy + Carl + Reggie = 360 ÷ 3 = 120

Total mass of the 4 children = 120

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

Answer(s): 30 kg