Spot the mistake in Roman numerals: worked example
22 October 2026
Roman numerals follow strict rules about the order of letters and how subtraction works. Spotting mistakes means checking if these rules are used correctly every time.
Worked example
Question: Spot the mistake in this Roman numeral and write it correctly.
IC
Step 1: Check the subtraction rule. Only I, X and C can be used to subtract, and only from the next two bigger numbers. I can only go before V (5) or X (10).
Step 2: In IC, I (1) is before C (100). That is not allowed. You cannot subtract 1 from 100.
Step 3: To write 99, split it into 90 and 9. 90 is XC (100 - 10). 9 is IX (10 - 1).
Step 4: Put them together: XCIX.
The correct answer is XCIX.
| Roman Numeral | Value |
|---|---|
| I | 1 |
| V | 5 |
| X | 10 |
| L | 50 |
| C | 100 |
| XC | 90 |
| IX | 9 |
| XCIX | 99 |
After this example, your child will know how to check if a Roman numeral follows the right rules. They can now look for similar mistakes in other questions and practise correcting them.
