OK, let's go one further on license plates...
What if no number or letter can appear more than once?
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?