The mass of 4 children, Opal, Marion, Jenson and Glen were taken. Only 3 children can be weighed at one time. Opal, Marion and Jenson weighed a total of 134 kg. Marion, Jenson and Glen weighed a total of 54 kg. Opal, Jenson and Glen weighed a total of 71 kg. Opal, Marion, and Glen weighed a total of 65 kg. What is the average mass of the 4 children?
Opal + Marion + Jenson = 134 --- (1)
Marion + Jenson + Glen = 54 --- (2)
Opal + Jenson + Glen = 71 --- (3)
Opal + Marion + Glen = 65 --- (4)

(1) + (2) + (3) + (4)
3(Opal + Marion + Jenson + Glen) = 134 + 54 + 71 + 65
3(Opal + Marion + Jenson + Glen) = 324
Opal + Marion + Jenson + Glen = 324 ÷ 3 = 108

Total mass of the 4 children = 108

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

Answer(s): 27 kg