2005 = MMV
Method.
M = 1000
MM = 1000 + 1000 = 2000
V = 5
MMV = 2000 + 5 = 2005
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
D1X
MCMLXXXV
MMDCCCLXXXVI
MMI
XLVI
XXI