The Romans used several different systems for writing numbers. Sometimes they wrote numbers like this: I II III IV V and other times they used the Greek numbers. Roman people didn't always write numbers the same way, either - people knew what you meant even if you did it a little differently. Here's a table showing all of the Roman numerals.
|IV (or IIII)
|IX (or VIIII)
So MMIII is 2003, and XXIV is 24, and CLVII is 157. (Putting a bigger number like V after a smaller number like I means "1 less than 5" or 4).
Try writing these numbers in Roman numbers (see the second page for the answers):