Answer :

Gibbs
An objects density is expressed as a combination of two units because the equation for density is Mass/Volume=Density. Therefore, both the mass and the volume retain a unit measure and when they are put together they create an expression that is a combination of two units such as kg/m^3 or g/cm^3. 
AL2006
Except for mass, length, time, and electric charge, EVERY quantity is
a combination of units ... of THOSE units in fact.  Hadn't you noticed ?