Not withstanding today's modern notation of Roman numerals in as much that the ancient Romans would have probably worked out the required calculations as follows:
MDCCLXXVI+IC = MDCCCLXXV => 1776+(100-1) = 1875
MDCCLXXVI+LXXXXVIIII = MDCCCLXXV => 1776+99 = 1875
MDCCLXXVI-IC = MDCLXXVII => 1776-(100-1) = 1677
MDCCLXXVI-LXXXXVIIII = MDCLXXVII => 1776-99 = 1677
Roman numerals are entirely inappropriate for doing such calculations. I believe the people in Roman times did such calculations on an abacus or something similar - which is basically similar to converting them to the Arabic numbers we use. If you really want to do it in Roman numerals - which is basically NOT a good idea - you would have to keep the thousands, hundreds, etc. separate, and handle carry (for addition) and borrowing (for subtraction).
When 9 is converted into Roman numerals it is IX which is an abridged version of VIIII and so the required calculations are as follows:-MDCCLXXVI+IX = MDCCLXXXV => 1776+(10-1) = 1785MDCCLXXVI+VIIII = MDCCLXXXV => 1776+9 = 1785MDCCLXXVI-IX = MDCCLXVII => 1776-(10-1) = 1767MDCCLXXVI-VIIII = MDCCLXVII => 1776-9 = 1767Note that in mathematics -(10-1) changes to 1-10QED
Nowadays 999 when converted into Roman numerals is now considered to be CMXCIX which does not quite easily lend itself for the purpose of arithmetical operations but the ancient Romans would have probably worked out the given three calculations as follows:-IM+DCCLXXVII = MDCCLXXVI => (1000-1)+777 = 1776DCCCCLXXXXVIIII+DCCLXXVII = MDCCLXXVI => 999+777 = 1776DCCCCLXXXXVIIII-DCCLXXVII = CCXXII => 999-777 = 222QED
It is now generally accepted that the modern way of expressing 1999 into Roman numerals is now MCMXCIX but the ancient Romans would have probably worked out the above as follows:- A: MDCCLXXVI+IMM = MMMDCCLXXV => 1776+(2000-1) = 3775 B: MDCCLXXVI+MDCCCCLXXXXVIIII = MMMDCCLXXV => 1776+1999 = 3775 C: MDCCCCLXXXXVIIII-MDCCLXXVI = CCXXIII => 1999-1776 = 223 The above calculations are fairly easy and straightforward but for more complicated calculations the Romans would have used an abacus counting device.
Under today's modern rules now governing the Roman numeral system the equivalent of 249 when converted into Roman numerals is now considered to be CCXLIX which does not lend itself quite easily to arithmetical operations but there exist credible evidence to suggest that the ancient Romans would have carried out the requested calculations as follows:-MDCCLXXVI+ICCL = MMXXV => 1776+(250-1) = 2025MDCCLXXVI+CCXXXXVIIII = MMXXV => 1776+249 = 2025MDCCLXXVI-ICCL = MDXXVII => 1776-(250-1) = 1527MDCCLXXVI-CCXXXXVIIII = MDXXVII => 1776-249 = 1527Note that in mathematics -(250-1) becomes -250+1 or as 1-250The above calculations were fairly simple and straight forward to work out but for more complicated calculations the Romans would make use of an abacus calculating device.QED
The modern way of expressing 49 into Roman numerals is now XLIX but the ancient Romans would have probably worked out the equivalent of 49 on an abacus counting frame as XXXXVIIII and then wrote it out as IL thus expediently working out the required calculations as follows:-LII+IL = CI => 52+(50-1) = 101LII-IL = III => 52-(50-1) = 3Note that in mathematics -(50-1) becomes -50+1 and that if we were to use the longer version of 49 in the above calculations the results would be exactly the same.QED
The rules as we know them today now governing the Roman numerals system had nothing to do with the Romans because they were introduced during the Middle Ages and as result of these rules the equivalent of 999 converted into Roman numerals is now considered to be CMXCIX which hardly lends itself quite easily for the purpose of mathematical operations but there exist credible evidence to show that the ancient Romans would have worked out the requested calculations as in the following formats:-MDCCLXXVI+IM = MMDCCLXXV => 1776+(1000-1) = 2775MDCCLXXVI+DCCCCLXXXXVIIII = MMDCCLXXV => 1776+999 = 2775MDCCLXXVI-IM = DCCLXXVII => 1776-(1000-1) = 777MDCCLXXVI-DCCCCLXXXXVIIII = DCCLXXVII => 1776-999 = 777Note that in mathematics -(1000-1) becomes 1-1000 and that the above calculations were fairly simple and straightforward to work out but for more advanced calculations the Romans would have used an abacus calculating device.QED
Notwithstanding today's rules now governing the Roman numeral system that were introduced during the Middle Ages inasmuch that there exist credible evidence to suggest that the ancient Romans would have worked out the required calculations as follows:-MDCCLXXVI+CLXXXXVIIII+CXXXXV = MMCXX => 1776+199+145 = 2120VXXCCMMI+ICC+VCL = MMCXX => (2001-225)+(200-1)+(150-5) = 2120Note that the above calculations are fairly simple and straightforward but for more complicated calculations the Romans would have made use of an abacus calculating device.QED
The modern way of expressing the equivalent of 444 into Roman numerals is now CDXLIV which does not lend itself quite easily for the purpose of calculations but there is historical evidence to suggest that the ancient Romans would have worked it out on an abacus counting device as CCCCXXXXIIII and then logically abridged it to IVLD in written format thus facilitating the speed and ease of the required calculations as follows:-MDCCLXXVI+IVLD = MMCCXX => 1776+(500-56) = 2220MDCCLXXVI+CCCCXXXXIIII = MMCCXX => 1776+444 = 2220MDCCLXXVI-IVLD = MCCCXXXII => 1776-(500-56) = 1332MDCCLXXVI-CCCCXXXXIIII = MCCCXXXII => 1776-444 = 1332Note that in mathematics -(500-56) changes to 56-500QED
Nowadays 249 when converted into Roman numerals is now considered to be CCXLIX but the ancient Romans probably worked out the required calculations as follows:-MDCCLXXVI+ICCL = MMXXV => 1776+(250-1) = 2025MDCCLXXVI+CCXXXXVIIII = MMXXV => 1776+249 = 2025MDCCLXXVI-ICCL = MDXXVII => 1776-(250-1) = 1527MDCCLXXVI-CCXXXXVIIII = MDXXVII => 1776-249 = 1527For more complicated calculations the Romans would have made use of an abacus calculating device.QED
The rules as we know them today now governing the Roman numeral system had absolutely nothing to do with the Romans because they were introduced during the Middle Ages and as a result of these rules the equivalent of 1999 when converted into Roman numerals is now considered to be MCMXCIX which hardly lends itself quite easily for the purpose of mathematical operations but notwithstanding the aforementioned inasmuch that there exist credible evidence to support the premiss that the ancient Romans would have carried out the requested calculations as in the following formats:-MDCCCCLXXXXVIIII+DCLXVI = MMDCLXV => 1999+666 = 2665IMM+DCLXVI = MMDCLXV => (2000-1)+666 = 2665MDCCCCLXXXXVIIII-DCLXVI = MCCCXXXIII => 1999-666 = 1333IMM-DCLXVI is the same as the above because IMM = MDCCCCLXXXXVIIIIFor more intricate and complicated calculations the Romans would have made use of an abacus calculating device.QED by David Gambell
Nowadays the modern way of converting 99 into Roman numerals is now considered to be XCIX but back in the past the ancient Romans would have worked it out on an abacus calculating board as LXXXXVIIII which can be logically abridged to IC thus facilitating the speed and ease of the required calculations as follows:-MDCCLXXVI+IC = MDCCCLXXV => 1776+(100-1) = 1875MDCCLXXVI+LXXXXVIIII = MDCCCLXXV => 1776+99 = 1875MDCCLXXVI-IC = MDCLXXVII => 1776-(100-1) = 1677MDCCLXXVI-LXXXXVIIII = MDCLXXVII => 1776-99 = 1677Note that in mathematics -(100-1) changes to 1-100 and that subtraction is achieved by cancelling out the numerals whereas addition is achieved by adding the numerals.QED