In original Content of 10 litre, there is 2 litre alcohol and 8 litre water.
Question ask to reduce concentration of alcohol to 5% (reduce concentration from 20% by 75%).
So lets assume X litre water is added.
So alcohol remains 2, water is 8+X and total content is 10+X.
Now 2*100/(10+X) = 5 => X = 30
very quickly: 2/(10+x)=1/20, x=30