It is 45.
Roman numerals are made up of 7 symbols: I = 1, V = 5, X =10, L = 50, C = 100, D = 500 and M = 1,000. For any given number, the largest symbol usually goes first, so for the year 2011, you get MMXI.
Symbols are repeated (iterated) to get multiples of the single symbol decimal values (being 1, 10, 100, 1000), so II = 2, III = 3, XX = 20, XXX = 30, MMM = 3,000 and so on.
When you get to the fourth repetition for any given symbol you use what is called a subtractive principle. This means that the lower valued symbol goes before the higher one to denote that this value should be deducted from the following symbol, so 4 becomes IV, 9 = IX, 14 = XIV.
So to answer your question, XLV means 50 less 10 plus 5 - or 45.
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