# Roman Numerals

Try the Roman Numeral Challenge.

Roman numerals are expressed by letters of the alphabet:

I=1 |

V=5 |

X=10 |

L=50 |

C=100 |

D=500 |

M=1000 |

There are four basic principles for reading and writing Roman numerals:

- 1. A letter repeats its value that many times (XXX = 30, CC = 200, etc.). A letter can only be repeated three times.
- 2. If one or more letters are placed after another letter of greater value, add that amount.
VI = 6 (5 + 1 = 6)

LXX = 70 (50 + 10 + 10 = 70)

MCC = 1200 (1000 + 100 + 100 = 1200)

3. If a letter is placed before another letter of greater value, subtract that amount. - a. Only subtract powers of ten (I, X, or C, but not V or L)
For 95, do NOT write VC (100 – 5).

DO write XCV (XC + V or 90 + 5) - b. Only subtract one number from another.
For 13, do NOT write IIXV (15 – 1 - 1).

DO write XIII (X + I + I + I or 10 + 3) - c. Do not subtract a number from one that is more than 10 times greater (that is, you can subtract 1 from 10 [IX] but not 1 from 20—there is no such number as IXX.)
For 99, do NOT write IC (C – I or 100 - 1).

DO write XCIX (XC + IX or 90 + 9) - 4. A bar placed on top of a letter or string of letters increases the numeral's value by 1,000 times.
XV = 15, = 15,000

IV = 4 (5 – 1 = 4)

XC = 90 (100 – 10 = 90)

CM = 900 (1000 – 100 = 900)

Several rules apply for subtracting amounts from Roman numerals:

One | I | Eleven | XI | Thirty | XXX |

Two | II | Twelve | XII | Forty | XL |

Three | III | Thirteen | XIII | Fifty | L |

Four | IV | Fourteen | XIV | Sixty | LX |

Five | V | Fifteen | XV | Seventy | LXX |

Six | VI | Sixteen | XVI | Eighty | LXXX |

Seven | VII | Seventeen | XVII | Ninety | XC |

Eight | VIII | Eighteen | XVIII | One hundred | C |

Nine | IX | Nineteen | XIX | Five hundred | D |

Ten | X | Twenty | XX | One thousand | M |