The relative density of a gas A with respect to another gas B is 2. The vapour density of the gas B is 20, the vapour density of the gas A is :

The relative density of a gas A when compre to another gas B is 2. 
So, the vapour density of the gas B is 20, then the vapour density of the gas A is 40.

$\frac{M_A}{M_B}=2$

$\Rightarrow M_A=2M_B$ or $D_A=2D_B$