A diet is to contain at least 80 units of minerals. Two foods \( F_1 \: and F_2\)are available. Food \( F_1 \) costs Rs.4 per unit food and \( F_2 \) costs Rs.6 per unit. A unit of food \( F_1 \) contains atleast 3 units of vitamin A and 4 units of minerals. A unit of food \( F_2 \)contains atleast 6 units of vitamin A and 3 units of minerals. Formulate this as a linear programming problem. Find the minimum cost for diet that consists of mixture of these two foods, and also meets the minimal nutritional requirements.