A mixture of 40 liters of milk $=36$ liters of Milk and 4 liters of water $=90:10$ ratio
Now the new mixture should be in the ratio of $80:20$
Hence $80\mathrm{\%}$ is equivalent to 36 liters (no addition of milk is done)
$100\mathrm{\%}$ is $(36/80{)}^{\ast }100=45$ liters of milk is present in the new mixture
Thus water shall be added $=(45-36-4)=5$ liters of water