10= X 11=XI 12=XII 13=XIII 14=XIV 15=XV 16=XVI 17=XVII 18=XVIII or XIIX 19=XIX 20=XX
In Roman numerals, XIIX is not a valid representation. The Roman numeral for 18 is XVIII, which is formed by combining the symbols for 10 (X), 5 (V), and 3 (III). The correct representation of 18 in Roman numerals follows the additive principle where smaller values are added to larger values to create the desired number.