
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. IV = 4 (5 – 1 = 4) XC = 90 (100 – 10 = 90) CM = 900 (1000 – 100 = 900) Several rules apply for subtracting amounts from Roman numerals: 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
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 
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