A paper mill produces two grades of paper viz., X and Y. Because of raw material restrictions, it cannot produce more than 400 tons of grade X paper and 300 tons of grade Y paper in a week. There are 160 production hours in a week. It requires 1.20 and 0.40 hours to produce a ton of grade X and Y papers. The mill earns a profit of Rs.200 and Rs.500 per ton of grade X and grade Y respectively. Formulate this as a linear programming problem.