Question

the relative density of mercury is 13.6 what does it mean?

Answer 1

Actually Relative density is the ratio of the density of fluid to the density of pure water at 4℃.

So,
If R.D. of mercury is 13.6 then it means

$\frac{density \: of \: mercury}{density \: of \: pure \: water \: } = r.d$

so density of
Mercury=r.d×density of pure water.

since density of pure water is fixed.

So we can directly find out density of mercury through it's R.D.