How do you write 45 in Roman numerals?

To write 45 in Roman numerals, split it into thousands, hundreds, tens and units and write each part: 40 (XL) + 5 (V). Put together, that gives XLV.

Roman numerals use seven letters: I = 1, V = 5, X = 10, L = 50, C = 100, D = 500 and M = 1000. A smaller letter in front of a larger one is subtracted (IV = 4, IX = 9, XL = 40, XC = 90, CD = 400, CM = 900); otherwise the values are added.