A farm is engaged in breeding pigs. The pigs are fed on various products grown on the farm. To ensure certain nutrient constituents X,Y and Z, It is necessary to mix two types of feeds A and B . One unit of product A contains 36 units of X, 3 units of Y and 20 units of Z. One unit of product B contains 6 units of X, 12 units of Y and 10 units of Z. The minimum requirement of X,Y and Z is 108 units, 36 units and 100 units respectively. Product A costs Rs. 20/unit and product B costs Rs 40/unit. Formulate this problem sa a linear programming problem to minimize the total cost.