Dana wants to make two types of dog treats. She has 10 cups of peanut butter and 12 cups of flour. Her dog bone treat recipe uses 3 cups of peanut butter and 2 cups of flour to make one tray. A tray of her oatmeal dog treat recipe uses 1 cup of peanut butter and 4 cups of flour. She plans to sell trays of dog treats at the town festival and charge 6 dollars for a tray of dog bone treats and 7 dollars for a tray of oatmeal treats. Dana wants to maximize her income from selling the dog treats. When writing constraints for the problem, which definition for the variables x and y can be used to determine the maximum?