XX in roman numeral is equivalent to 20.
Basic rules for writing Roman Numerals:
When a symbol appears after a larger symbol it is added
Example: VI = V + I = 5 + 1 = 6
If the symbol appears before a larger symbol it is subtracted
Example: IX = X - I = 10 - 1 = 9
Don't use the same symbol more than three times in a row.
Numbers greater than 1,000 are formed by placing a dash over the symbol, meaning "times 1,000", but these are not commonly used.
The Roman Numeral Symbols:
See this Numerals Conversion Chart to translate large numbers into Roman Numerals.
Basic Combinations:
Roman Numerals via kwout