How do you write 5th in roman numbers?

Answer 1

As an ordinal fifth, you'd just use V. As in King Henry V (read as King Henry the fifth).

As a fraction, you can't write 1/5th in Roman numerals. Roman fractions were purely duodecimal (base-12) in nature, and could only handle 1/12ths. Thus you could have 1/2, 1/3, 1/4 and 1/6, but not 1/5. You would need a sexagesimal system (base-60) to handle 1/2, 1/3, 1/4, 1/5 and 1/6. Although such systems existed then (long before Roman numerals were conceived), the Romans did not use them.

Note that we still use sexagesimal systems to this day. The 12-hour clock (60 minutes per hour, 60 seconds per minute); 180 degrees in a triangle's angles and 360 degrees in a circle, etc. We still use it for the same reasons the Anciant Babylonians used it: 60 is the smallest number evenly divisible by 2, 3, 4, 5 and 6.

Answer 2

The Roman numeral for 5th is V.