If repetition of the same four digits is not allowed (for example, 5555), how many possible combinations are there?

If repetition of the same four digits is not allowed (for example, 5555), how many possible combinations are there? Example 5: In recent years, the state of California issued license plates using a combination of one letter of the alphabet followed by three digits, followed by another three letters of the alphabet. How many different license plates can be issued using this configuration?