How do you actually work out the sum of 1981 and 19 in Roman numerals knowing that it must be MM?

Answer 1

As a consequence of changes made to the real rules governing the Roman numeral system in the Middle Ages today we would write out the equivalent of 1981 and 19 into Roman numerals as MCMLXXXI and XIX respectively which makes addition of them in some kind of logical progression quite difficult.

But during the Roman era 1981 and 19 were calculated as MDCCCCLXXXI and XVIII respectively and were probably simplified to XXMMI and IXX which makes addition straightforward as follows:-

XXMMI+IXX = MM (2001-20)+(20-1) = 2000

Alternatively:-

MDCCCCLXXXI+XVIIII = MM (1981+19 = 2000)