Under today's rules 99,999 written in Roman numerals is: (XCIX)CMXCIX
Numerals in brackets indicate multiplication by a thousand.
Hence:
(XCIX) = 99*1000 = 99,000
CMXCIX = 999
But today's rules governing the Roman numeral system were introduced during the Middle Ages so it's quite possible that the Romans themselves would have calculated the number 99,999 on a abacus counting device as (LXXXXVIIII)DCCCCLXXXXVIIII and then simply wrote it down as I(C) meaning minus 1 plus 100,000 which equals 99,999.
IC (C with line over it).
42 in Roman numerals is XLII.
MMMM
LXXXIII
DCCXXXII
DCXXV
MM
MCMLXXXVII
22 in roman numbers is "XXII"
It is: 2000 = MM
It is: 753 = DCCLIII
In today's terms it is written out as (XCIX)CMXCIX but the Romans themselves would have probably wrote it out simply as I(C) which means -1+100,000 = 99,999