A professor wants to arrange his books on a shelf. he has 30 books and only space on the shelf for 20 of them. how many different 20-book arrangements can he make using the 30 books? this is an example of a problem that can be solved using which method?

This is an example of a problem that can be solved using combination. 

The number of possible arrangements is 
     [tex]P\left(30,20\right)=\frac{30!}{\left(30-20\right)!\times 20!}=30,045,015[/tex]

There are 30,045,015 possible 20-book arrangements.