Roman Numerals

[Chadonis]

How did the Romans add, subtract or multiply roman Numerals?

[dundurn]

Mostly they didn't. The Roman numerals we are familiar with were used to record the answers, but weren't used in the calculations. Calculations were performed using tallies (the 'five-barred gate' type we still use today), ready reckoners (write down the results of frequently used calculations and then just look up the answer when required) and the abacus. The Roman abacus didn't use beads on wires but little balls (calculi) in grooves on a board. Functionally it was identical to the modern Japanese abacus which is (in trained hands) as fast as a calculator for day to day arithmetic. There were also the good old-fashioned tools of mental arithmetic, times tables and fingers!