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Relative density = \frac{density of substance}{density of water}
it is a dimensionless quantity 
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Relative density of a substance=(Density of any substance)/(Density of water at 4 degree Celsius)
so, Relative Density is related with the density of water, taken as 1 g/cm³.
Since RD is a ratio , it is a unit less  quantity.
Example=>The RD of Mercury is 13.6 .
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