A cube of side 20 cm has mass 16 kg. Its relative density is 

Density of object = mass/volume
                             =16000g / volume
volume of cube = s³ =20³ = 8000cm³
                             =16000/8000
                             = 2g/cm³
 Relative density  = density of substance/density of water
                             =(2g/cm³)/(1g/cm³)
                             =2
[relative density has no unit as it compares quantities of similar unit] 

Please give me a thumbs up if it was helpful :)