Indian Institute of Foreign Trade 2014 Solved Paper

A milk vendor sells 10 litres of milk form a can containing 40 litres of pure milk to the 1${}^{\mathrm{st}}$ customer. He then adds 10 litres of water to the milk can. He again sells 10 litres of mixture to the 2${}^{\mathrm{nd}}$ customer and then adds 10 litres of water to the can. Again he sells 10 litres of mixture to the 3${}^{\mathrm{rd}}$ customer and then adds 10 litres of water to the can and so on. What amount of pure milk will the 5th customer receive?