A milkman adds 10 litres of water to 90 litres of milk. After selling $$\frac{1}{5}^{th}$$ of the total quantity, he adds water equal to the quantity he sold. The proportion of water to milk he sells now would be:

Initially Milkman has 90 liters of milk
Now amount of water added = 10 liters 
So total quantity becomes : 100 liters 
He sold 1/5 of quantity 
so he sold 2 liters of water and 18 liters of milk (A total of 20 liters is sold )
Now he is left with 10-2 = 8 liters of water  and 90-18 = 72 liters of milk.
Now he adds 20 liters of water
so the amount of milk and water with him will be : 72 and 28 respectively.