OK, let's go one further on license plates...

What if no number or letter can appear more than once?

ex:

Let's think this out -- one slot at a time:

We'll do all the numbers first...

No restrictions yet.
 

Now, let's say that a 5 went in the first slot:

So, a 5 cannot go here...  That leaves 9 possibilities:

Now two numbers have been used...  It might be

8 possibilities here:

                                                    and 7 possibilities here

Now, let's do the letters...  No letter can appear more than once...

Can you finish it off?