The mass of 4 children, Sarah, Barbara, Cody and Vincent were taken. Only 3 children can be weighed at one time. Sarah, Barbara and Cody weighed a total of 88 kg. Barbara, Cody and Vincent weighed a total of 111 kg. Sarah, Cody and Vincent weighed a total of 84 kg. Sarah, Barbara, and Vincent weighed a total of 137 kg. What is the average mass of the 4 children?
Sarah + Barbara + Cody = 88 --- (1)
Barbara + Cody + Vincent = 111 --- (2)
Sarah + Cody + Vincent = 84 --- (3)
Sarah + Barbara + Vincent = 137 --- (4)

(1) + (2) + (3) + (4)
3(Sarah + Barbara + Cody + Vincent) = 88 + 111 + 84 + 137
3(Sarah + Barbara + Cody + Vincent) = 420
Sarah + Barbara + Cody + Vincent = 420 ÷ 3 = 140

Total mass of the 4 children = 140

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

Answer(s): 35 kg