The Roman numeral equivalent of the number 42 is XLII. The Roman numeral system uses letters to denote numbers. The basic Roman numerals used to write numbers up to 100 are as follows: I equals one, V equals five, X equals 10, L equals 50 and C equals 100.
Numbers are added to or subtracted from the basic numbers to arrive at all other numbers. No letter is allowed to appear more than three times in succession. For the number 42, the forty is written as XL, which means 50 minus 10, and the two is written as II. When the X appears directly before the L, 10 is subtracted from 50, but when it appears directly after the L, 10 is added to 50.