I think PC and CP defines the same thing right? In the end you will be buying physics and chemistry books??
A student decided to buy books at a bookstore. He wants to buy at least 2 books. With the amount of money he has, he can only buy a max of 3 books. There are three shelves in the book store, each shelves with the following subject : physics, chemistry, and biology. How many possible are there to make a successful purchase? (repetition is allowed)