Roman numerals conversion and chart
Roman Numerals Conversion 
I = 1 
C = 100 
V = 5 
D = 500 
X = 10 
M = 1000 
L = 50 


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Roman Numerals120
Roman Numerals Conversion Rules:
The letters should be arranged from the one with the largest value to the one with the smallest.
Each letter's value is added to the previous ones.
Only powers of ten (I, X, C, M) can be repeated.
Do not repeat any letter more than three times in a row.
Because of the preceding rule, certain numbers must be written using subtraction. In this case, a letter with a smaller value precedes one with a larger value and the value of the smaller is subtracted from the larger. The result is then added to the rest of the letters. The following rules apply to subtraction:
Only powers of ten (I, X, C, M) can be subtracted.
The smaller letter must be 1/5th (onefifth) or 1/10th (onetenth) the larger one.
The smaller letter must be either the first letter or preceded by a letter at least ten times greater than it.
If another letter follows the larger one, it must be smaller than the number preceding the larger one.
[from: http://home.hiwaay.net/~lkseitz/math/roman/numerals.shtml]