Solve the given linear programming problem graphically:
Nutrition: A dietitian is to prepare two foods to meet certain requirements. Each pound of Food I contains 100 units of vitamin C, 40 units of vitamin D, and 10 units of vitamin E and costs 20 cents. Each pound of Food II contains 10 units of vitamin C, 80 units of vitamin D, and 5 units of vitamin E and costs 15 cents. The mixture of the two foods is to contain at least 260 units of vitamin C, at least 320 units of vitamin D, and at least 50 units of vitamin E. How many pounds of each type of food should be used to minimize the cost?