What if we have the eight books...  But, we can only put three of them on the shelf?  How many arrangements are possible?

Let's figure it out!

** It's ALWAYS better to think than to just try to slam through a memorized formula!

Here are the books and the slots:
 

 

 

How many choices do we have for this slot?

Let's say we put book C in slot 1...  How many choices are left?

How many are left?
 

That wasn't bad at all!

 

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TRY IT:

If you have 10 books and you want to arrange 4 of them on a bookshelf, how many ways can you do it?

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