A milkman has $80\%$ of milk in his stock of $800$ litres of adulterated milk. How much $100\%$ milk to be added to give certain purity? Problem: A milkman has $80\%$ of milk in his stock of $800$ litres of adulterated milk. How much $100\%$ milk to be added to it so that the purity of milk is between $90\%$ and $95\%$ 
Let $x$ litres $100\%$ pure  milk need to be added in $800$ litres of milk. 
Please suggest further how to proceed not getting any idea on this. 

Here you have 800 l total 
640 milk 
160 water. 
Make that 160 10 percent. 
So total 1600. 
Add 1600-640. That is 960l of pure milk 
