The mass of 4 children, Shannon, Kimberly, Simon and Nick were taken. Only 3 children can be weighed at one time. Shannon, Kimberly and Simon weighed a total of 61 kg. Kimberly, Simon and Nick weighed a total of 142 kg. Shannon, Simon and Nick weighed a total of 143 kg. Shannon, Kimberly, and Nick weighed a total of 134 kg. What is the average mass of the 4 children?
Shannon + Kimberly + Simon = 61 --- (1)
Kimberly + Simon + Nick = 142 --- (2)
Shannon + Simon + Nick = 143 --- (3)
Shannon + Kimberly + Nick = 134 --- (4)

(1) + (2) + (3) + (4)
3(Shannon + Kimberly + Simon + Nick) = 61 + 142 + 143 + 134
3(Shannon + Kimberly + Simon + Nick) = 480
Shannon + Kimberly + Simon + Nick = 480 ÷ 3 = 160

Total mass of the 4 children = 160

Average mass of the 4 children
= 160 ÷ 4
= 40

Answer(s): 40 kg