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. 

$80\%$ is milk out of $800$ litres. That gives - you have $640$ litres of pure milk.  Now, $640+x\over {800+x}$$>0.9$  That gives $x>800$ litres. Since, it should also be less than $0.95$ , You get $x<1200$ litres.
