What is the roman numeral for the 20th century?

Answer 1

20th is a linguistic ordinal number (first, second, third, etc). It cannot be represented as a Roman numeral which is strictly cardinal.

Using cardinals, one way it might be represented is: "Century XX"

Although the new millennium was celebrated on 1st January 2000, the 20th century didn't officially end until January 1st 2001. The year 2000 was the final year of the 20th century (years 1901-2000), and the first of the new millennium (years 2000 - 2999). Thus another way to notate the 20th century might be MCMI-MM (1901-2000).