Verifying Solidity/Hollowness of a Copper Object

[jpfelgueiras]

I got a small conserning how to determine if a object is solid or hollow.
"Verify if a copper object with density of 8,96 g/m^3, which weights 2,8N on the air and when submersive on water (p=998 kg/m^3) weights 1,3N, is it solid or hollow?"

 

[Doc Al]

How could you solve for the density of the object? (Note that the density given for copper should be in g/cm^3, not g/m^3.)