2Cu+O2=2CuO

C+O2=CO2

The mass of the remaining solid is the mass of the original mixture, so the mass of carbon in the original mixture is equal to the reaction The mass of oxygen in the remaining solid after the operation

The mass ratio of carbon and copper in the original mixture is 16:64=1:4

The mass fraction of carbon in the original mixture is 16/80*100 %=20%.