Amanda likes to eat apples and bananas with her lunch between classes. She also keeps a close eye on her spending each week since she is in college and cannot afford the time to work too much. She has budgeted a certain amount of money each week for apples and bananas. If she spends all of this budget on apples, she can purchase 6 apples. If she spends all of the budget on bananas, she can afford 9 bananas. What is Amanda's opportunity cost of an apple?