The mass of 4 children, Sabrina, Cindy, Ethan and Xavier were taken. Only 3 children can be weighed at one time. Sabrina, Cindy and Ethan weighed a total of 146 kg. Cindy, Ethan and Xavier weighed a total of 53 kg. Sabrina, Ethan and Xavier weighed a total of 118 kg. Sabrina, Cindy, and Xavier weighed a total of 55 kg. What is the average mass of the 4 children?
Sabrina + Cindy + Ethan = 146 --- (1)
Cindy + Ethan + Xavier = 53 --- (2)
Sabrina + Ethan + Xavier = 118 --- (3)
Sabrina + Cindy + Xavier = 55 --- (4)

(1) + (2) + (3) + (4)
3(Sabrina + Cindy + Ethan + Xavier) = 146 + 53 + 118 + 55
3(Sabrina + Cindy + Ethan + Xavier) = 372
Sabrina + Cindy + Ethan + Xavier = 372 ÷ 3 = 124

Total mass of the 4 children = 124

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

Answer(s): 31 kg