The mass of 4 children, Opal, Eva, Perry and Fred were taken. Only 3 children can be weighed at one time. Opal, Eva and Perry weighed a total of 53 kg. Eva, Perry and Fred weighed a total of 121 kg. Opal, Perry and Fred weighed a total of 123 kg. Opal, Eva, and Fred weighed a total of 147 kg. What is the average mass of the 4 children?
Opal + Eva + Perry = 53 --- (1)
Eva + Perry + Fred = 121 --- (2)
Opal + Perry + Fred = 123 --- (3)
Opal + Eva + Fred = 147 --- (4)

(1) + (2) + (3) + (4)
3(Opal + Eva + Perry + Fred) = 53 + 121 + 123 + 147
3(Opal + Eva + Perry + Fred) = 444
Opal + Eva + Perry + Fred = 444 ÷ 3 = 148

Total mass of the 4 children = 148

Average mass of the 4 children
= 148 ÷ 4
= 37

Answer(s): 37 kg