A mixture contains milk and water in the ratio 4 : 3 (by volume). If 4 litres water is added to it, then the ratio becomes 1: 1. What is the quantity of milk in the initial mixture?

Initially mixture  is 4:3 

After adding 4 lit water it becomes 1:1

It means quantity of milk is remain unchanged and water is changed in mixture 

4:3  ......(1)

1:1  $$\times 4$$ (to equal the milk quantity )

= 4:4 ......(2)

Now milk ratio is same and water is changed after adding water 

difference in the ratio (1) and  (2) = 1: 0 unit

1 unit -> 4 lit 

So in 4:3 ratio

 Milk =$$ 4\times 4$$= 16 litre

Water = $$ 3\times 4$$= 12 litre