In 20 litres mixture of acid, the ratio of acid and water is 2 : 3. How much water should be added to the mixture so that the ratio of the acid to water becomes 2 : 5 ?

8 litres

Total mixture $=20$ litres.

Ratio of acid : water $=2:3$.

Quantity of acid $=\frac{2}{5}\times20=8$ litres.

Quantity of water $=\frac{3}{5}\times20=12$ litres.

Let $x$ litres of water be added.

New water $=12+x$ litres, acid remains $8$ litres.

New ratio:

$\frac{8}{12+x}=\frac{2}{5}$

$40=24+2x$

$2x=16$

$x=8$

final answer: $8$ litres of water should be added