The variance of 20 observations is 5. If each one of the observations is multiplied by 2, then the variance of the resulting observations is 

 If each one of the observations is multiplied by 2 then mean is doubled

Now, varience $={\sigma }^2=\frac{(\mathrm{X}-\mu {)}^2}{\mathrm{n}}$

Now ,both X and $\mu$ is doubled then varience will become four times

Then,the new variance will be four times the old variance which is 

$=4\times 5$

$=20$