Question 548287
Okay so we have a total of n=10 books and we want to choose 4 of them to bring with us.   Order does not matter (it doesn't matter what order you picked the books, only that you picked them) so we are dealing with combinations .  Therefore there are 
10C4 different sets.  This can be simplified using the combinations formula, but I will leave that part up to you:) 

Let me know if you are still unsure.