A jar contains x litres of milk. A seller withdraws 25% of it and sells it at Rs. 20 per litre. Then he replaces it with water. He repeats the process total three times and every time while selling, the selling price reduces by Rs. 2. After this process, the milk left in the mixture is only 108 litres, so he decided to sell the entire mixture at Rs. 15 per litre. How much profit did he earn if bought milk at Rs. 20 per litre?