The mass of 4 children, Cindy, Jade, Billy and Pierre were taken. Only 3 children can be weighed at one time. Cindy, Jade and Billy weighed a total of 120 kg. Jade, Billy and Pierre weighed a total of 83 kg. Cindy, Billy and Pierre weighed a total of 108 kg. Cindy, Jade, and Pierre weighed a total of 49 kg. What is the average mass of the 4 children?
Cindy + Jade + Billy = 120 --- (1)
Jade + Billy + Pierre = 83 --- (2)
Cindy + Billy + Pierre = 108 --- (3)
Cindy + Jade + Pierre = 49 --- (4)

(1) + (2) + (3) + (4)
3(Cindy + Jade + Billy + Pierre) = 120 + 83 + 108 + 49
3(Cindy + Jade + Billy + Pierre) = 360
Cindy + Jade + Billy + Pierre = 360 ÷ 3 = 120

Total mass of the 4 children = 120

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

Answer(s): 30 kg