Roman numbers are expressed by letters. Here is the list of letters used, with thier equivalent values :
| i | 1 |
| v | 5 |
| x | 10 |
| l | 50 |
| c | 100 |
| d | 500 |
| m | 1000 |
2. Repeat letters
You can repeat a letter, but not more than three times, and its value is added as many times as it repeats.
| ii | 2 |
| xx | 20 |
| xxx | 30 |
| ccc | 300 |
3. v, l and d are never repeated
Means you can't write vv, vvv, ll, lll, dd, ddd.
4. Addition
If a letter of smaller value is written just after a letter of greater value, its value gets added to the value of greater letter.
| vi | 5+1 | 6 |
| xi | 10+1 | 11 |
| xii | 10+2 | 12 |
| xxxiii | 30+3 | 33 |
5. Subtraction
If a letter of smaller value is written just before a symbol of greater value, its value is subtracted from the value of the greater symbol.
| iv | 5-1 | 4 |
| ix | 10-1 | 9 |
| xl | 50-10 | 40 |
| xc | 100-10 | 90 |
6. Don't repeat a letter of smaller value before a letter of greater value.
Here are some examples:
| Number | Correct | Wrong |
| 8 | viii | iix |
| 18 | xviii | xiix |
| 80 | lxxx | xxc |
7. Don't repeat a letter of greater value after a letter of smaller value.
Here are some examples:
| Number | Correct | Wrong |
| 19 | xix | ixx |
| 18 | xviii | iixx |
8. v, l and d are never written before a letter of greater value.
9. i can be subtracted from v and x only.
x can be subtracted from l and c only.
c can be subtracted from m only.