non-trivial arithmetic

The classical/medieval trivium consisted of grammar, logic, and rhetoric. The quadrivium consisted of arithmetic, geometry, music, and astronomy. I always joked that when you're stuck with Roman numerals, doing arithmetical calculations is an advanced subject. In my recent re-dipping into number theory, I have learned that arithmetic was synonymous with number theory, or vice versa, and many number theoretic proofs are part of Euclid's elements. So the subject of the quadrivium may have been more advanced than I thought, if not more useful. (Pity the manor whose lord made plans based on the properties of perfect numbers.)

(How does one do calculations with Roman numerals? "Use an abacus", I assume.)