The mass of 4 children, Zoe, Pamela, Xavier and Ben were taken. Only 3 children can be weighed at one time. Zoe, Pamela and Xavier weighed a total of 113 kg. Pamela, Xavier and Ben weighed a total of 87 kg. Zoe, Xavier and Ben weighed a total of 66 kg. Zoe, Pamela, and Ben weighed a total of 58 kg. What is the average mass of the 4 children?
Zoe + Pamela + Xavier = 113 --- (1)
Pamela + Xavier + Ben = 87 --- (2)
Zoe + Xavier + Ben = 66 --- (3)
Zoe + Pamela + Ben = 58 --- (4)

(1) + (2) + (3) + (4)
3(Zoe + Pamela + Xavier + Ben) = 113 + 87 + 66 + 58
3(Zoe + Pamela + Xavier + Ben) = 324
Zoe + Pamela + Xavier + Ben = 324 ÷ 3 = 108

Total mass of the 4 children = 108

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

Answer(s): 27 kg