Question

what is relative density

Answer 1

relative density of a substance is the ratio of the density of the substance to the density of water at 4 degree C .
   R.D. of a substance = density of substance / density of water at 4 degree C.

Answer 2

Its the ratio of an object's density to that of water. As it is a ratio it does not have any SI unit. We calculate relative density because it is convinient to exprees an object's density with comparsion to water. By relative density, we can tell whether an object will float or sink in water