I see guides online but that doesn't necessarily mean the Romans used this system.
I found some previous answers to similar questions:
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We have gotten this question before, and in addition to what u/lumberjerk found and posted below, we have a section of our FAQ that may be of interest: https://www.reddit.com/r/Askhistorians/wiki/science#wiki_using_roman_numerals_to_do_mathematics
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