Imagine a question and stream-of-consciousness explanation for which this is the answer: 11
Question: How many shirts should Shenny pack for her next vacation if she's planning to use the same shirt when departing on Monday and returning on Sunday and two different shirts each other day?
Stream-of-consciousness: Between Monday and Sunday there are 7 days. If she will be using the same shirt for 2 days then she will need clothes for 7 - 2 = 5 days. Because she wants to use 2 different shirts each of those 5 days then she needs 2 * 5 = 10 shirts. In total she will need 10 shirts between Tuesday and Saturday and 1 shirt for Monday and Sunday = 11 shirts.