I = one, V= 5, X =10, L=50, C = 100, D = 500, M=1000. 9 for example is 1 less than 10 therefore it is written IX, 11 is one more than 10= XI.
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