Oh, dude, 1998 in Roman numerals is MCMXCVIII. It's like the year you were rocking out to the Backstreet Boys and playing with your Tamagotchi. So retro, right?
The Roman numerals are XLV.
18 in roman numerals looks like XVIII
20 in Roman numerals is XX.
300 would be represented by 'CCC' in roman numerals.
MCMLXXXIX
DCLXVI
MCMLII
XXI
XL
MCMLXV
MMI
XLVI