The relative density of silver is 10.8, and the density of water is ${10}^{3\mathrm{ }}\mathrm{kg}/{\mathrm{m}}^3$. What is the density of silver?

We know that, 

Relative density of a substance = density of that substance/density of water
$10.8=\frac{\mathrm{density} \mathrm{of} \mathrm{silver}}{{10}^3}$.

$\mathrm{density} \mathrm{of} \mathrm{silver} =10800 \mathrm{kg}/{\mathrm{m}}^3$
Therefore, density of silver$=10800\mathrm{kg}/{\mathrm{m}}^3$.