A diet is to contain at least 4000 units of carbohydrates, 500 units of fat and 300 units of protein. Two foods `F_1` and `F_2` are available. Food `F_1` costs 2 Rs. per unit and food `F_2` costs 4 Rs. per unit. A unit of food `F_1` contains 10 units of carbohydrates, 20 units of fat and 15 units of protein. A unit of food `F_2` contains 25 units of carbohydrates, 10 units of fat and 20 units of protein. Find the minimum cost for a diet that consists of a mixture of these two foods and also meets the minimum requirements.