A license plate consists of a letter, followed by three numbers, followed by another letter. Due to possible confusion, the letters I and O are not used. Repetition is not
allowed.
How many different license plates are possible?

This is one single number that's slightly smaller than 400 thousand.

Explanation:

There are 26 letters in the english alphabet. We don't use letters i or o, so we have 26-2 = 24 choices for that first slot.

Then we have 10 choices for the second slot because there are 10 single digits 0 (zero) through 9.

After making the selection for the second slot, we have 10-1 = 9 choices left for the third slot. The fourth slot has 10-2 = 8 digits to pick from. The subtraction occurs because we cannot reuse the digits.

Finally, the last slot will be a letter. We have 24-1 = 23 letters left to pick from.

Overall, we have 24*10*9*8*23 = 397,440 different license plates possible.

Extra info (optional section)

You may be wondering "why do we multiply those values?". Well consider a simple example of having only 2 slots instead of 5.

Let's say the first slot is a letter A to Z, excluding letters i and o. The second slot will be the digit from 0 through 9.

If you make a table with 24 rows and 10 columns, then you'll have 24*10 = 240 cells overall. Notice the multiplication here. The 24 rows represent each letter, and the 10 columns are the digit numeric digits.

Each inner cell is a different two character license plate. For example A5 is one such plate. This idea can be extended to have 5 characters.