[Coda]

X * III + I

It's actually not too complicated. For addition, you just stick all the letters together and then fix up cases where it breaks the rules (e.g. IIII -> IV). For subtraction, you take out matching letters, and you can replace a letter with multiple copies of a smaller letter (e.g X -> VV) if you need to.

Multiplication is a bunch of replacements. You multiply the whole first number by each letter in the second number, so X * III is X, X, and X, which you stick together to be XXX. Once you're done with that, you go do the same fixup that you had to do for addition. You only have to memorize a fairly small times table (V * V = XV, for example).