A dietitian is designing a daily diet that is to contain at least 60 units of protein, 40 units of carbohydrates, and 120 units of fat. The diet is to consist of two types of foods. One serving of food A contains 30 units of protein, 10 units of carbohydrates, and 20 units of fat and costs $3. One serving of food B contains 10 units of protein, 10 units of carbohydrates, and 60 units of fat and costs $1.50. Design the diet that provides the daily requirements at the least cost.