Help! Joe goes to a bookstore with the intent of buying 4 books about American History and then putting them on his new book shelf. The book has 20 books on American History. How many different ways are there for Joe to select 4 books and then arrange them on his bookshelf? I used nCr and came up with 4485 ways to chose 4 books out of 20. Do I use that # to nCr or nPr from this point? Is this math question looking for 2 different answers? or am I making it too difficult?