The mass of 4 children, Fiona, Mary, Perry and Seth were taken. Only 3 children can be weighed at one time. Fiona, Mary and Perry weighed a total of 72 kg. Mary, Perry and Seth weighed a total of 149 kg. Fiona, Perry and Seth weighed a total of 60 kg. Fiona, Mary, and Seth weighed a total of 139 kg. What is the average mass of the 4 children?
Fiona + Mary + Perry = 72 --- (1)
Mary + Perry + Seth = 149 --- (2)
Fiona + Perry + Seth = 60 --- (3)
Fiona + Mary + Seth = 139 --- (4)

(1) + (2) + (3) + (4)
3(Fiona + Mary + Perry + Seth) = 72 + 149 + 60 + 139
3(Fiona + Mary + Perry + Seth) = 420
Fiona + Mary + Perry + Seth = 420 ÷ 3 = 140

Total mass of the 4 children = 140

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

Answer(s): 35 kg