A diet is to contain at least 80 units of vitamin A and 100 units of minerals. Two foods \( F_1 \: and F_2\) are available . Food \( F_1 \) costs Rs. 4 perunit and \( F_2\) costs Rs. 6 per unit. One unit of food \( F_1 \) contains 3 units of vitamin A and 4 units of minerals. One unit of food \( F_2 \) contains 6 units of vitamin A and 3 units of minerals. Formulate this as a linear programming problem and find graphically the minimum cost for diet that consists of mixture of these two foods and also meets the minimal nutritional requirements.