Let’s have a pretty quick maths lesson here. Apologies for historical inaccuracy but the gist is right.
The wonderful thing about our counting system is that it embodies something called a number base. Most people don’t understand what the number 87 means. It’s simple: it means 8*101 + 7*100. Every time you add a digit on the left, it means you’re multiplying it by 10n, where n starts from 0. This is called base-10. The reason we use it over base 35 is that we happen to have 10 fingers. Computer use base-2 because they can represent their symbols as ‘on’ and ‘off’ and struggle to add a third. Sometimes we use base-16 because it happens to map quite neatly to base-2 and is easier to read. But whatever we’re doing with numbers, *we are always using bases*.
Because of the way based numbers can be split by their digits, we can, for example, easily add them up column by column.
Roman numerals don’t work like this. Roman numerals don’t have any underlying logic. They do not represent a number base. Trying to do arithmetic on roman numerals is stupid because there aren’t any useful consistent rules that follow from the numbering system. Roman numerals are not fit for any purpose whatever. The reason the Roman empire collapsed was that they couldn’t keep their accounts straight*
The ancient Greeks and the ancient Indians were pretty hot on maths (I think it was Indians anyway) and they had all this figured out. The Romans, not so much. That’s why the Romans fought battles with an endless supply of soldiers, whereas the Greeks had catapults. There’s a reason all our mathematical symbols are named after Greek letters.
Michael Gove, go back to school.
* This may not be true, but I’m sure it must have been a factor.