A mixture contains milk and water in the ratio 5 : 2 If 4 litres of water is added to the mixture, the ratio becomes 5 : 4. What is the quantity of milk in the given mixture?

10 litres 

     Milk   :  Water 

       5     :     2

After adding 4 litres of water, ratio becomes

       5     :     4

Thus, 2R = 4 (difference in ratio of water, because ration of milk is equal)

          R = 2

Quantity of milk = 5 x 2 = 10 litres