The mass of 4 children, Jen, Sarah, Luis and Mark were taken. Only 3 children can be weighed at one time. Jen, Sarah and Luis weighed a total of 123 kg. Sarah, Luis and Mark weighed a total of 109 kg. Jen, Luis and Mark weighed a total of 81 kg. Jen, Sarah, and Mark weighed a total of 59 kg. What is the average mass of the 4 children?
Jen + Sarah + Luis = 123 --- (1)
Sarah + Luis + Mark = 109 --- (2)
Jen + Luis + Mark = 81 --- (3)
Jen + Sarah + Mark = 59 --- (4)

(1) + (2) + (3) + (4)
3(Jen + Sarah + Luis + Mark) = 123 + 109 + 81 + 59
3(Jen + Sarah + Luis + Mark) = 372
Jen + Sarah + Luis + Mark = 372 ÷ 3 = 124

Total mass of the 4 children = 124

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

Answer(s): 31 kg