A mixture of 50 liters of milk and water contains 10% of water. How much water must be added to make the water 20% in the new mixture?

Quantity of water in given mixture
=10 / 100 x 50=5 litres.

Quantity of milk in given mix.
= 50 - 5= 45 litres.

Let x litres of water be added to it.
Milk : 45 litres, water (5 + x) litres
Total mix = (50 + x) litres
(5+x)/ (50+ x ) = 20/100
⇒ (5 + x)5 = 50 + x
⇒ 4x = 25
⇒ x= 6.25
6.25 litres of water must be added.