The mass of 4 children, Joelle, Zara, Cole and Elijah were taken. Only 3 children can be weighed at one time. Joelle, Zara and Cole weighed a total of 79 kg. Zara, Cole and Elijah weighed a total of 147 kg. Joelle, Cole and Elijah weighed a total of 141 kg. Joelle, Zara, and Elijah weighed a total of 53 kg. What is the average mass of the 4 children?
Joelle + Zara + Cole = 79 --- (1)
Zara + Cole + Elijah = 147 --- (2)
Joelle + Cole + Elijah = 141 --- (3)
Joelle + Zara + Elijah = 53 --- (4)

(1) + (2) + (3) + (4)
3(Joelle + Zara + Cole + Elijah) = 79 + 147 + 141 + 53
3(Joelle + Zara + Cole + Elijah) = 420
Joelle + Zara + Cole + Elijah = 420 ÷ 3 = 140

Total mass of the 4 children = 140

Average mass of the 4 children
= 140 ÷ 4
= 35

Answer(s): 35 kg