Explanation:
For
6
books, that means we have
6
spots to put them in.
Our first book has
6
spots it can be in.
Our second book will now have
5
, since one is taken up by the first spot.
The next will have
4
, then
3
,
2
, and
1
. Thus, we can model the number of arrangements as
6
⋅
5
⋅
4
⋅
3
⋅
2
⋅
1
=
6
!
=
720
Therefore, we have
720
ways of arranging
6
books on the shelf.
Hope this helps!
