When I was at primary school, we were taught the rules for Roman numerals. This was not just a theoretical study: in those days, many clocks used Roman numerals (though often they broke the rules by using IIII for 4), and they were also used for dates, especially in the credits of films – you had to be efficient at decoding Roman numerals to read the date as the credits scrolled past.
Then the teacher asked us what number was represented by the Roman numeral IXL. The pupils were puzzled: was it XL−I? (but that should be XXXIX); was it L−IX? (but that should be XLI).
Eventually the teacher revealed the answer, “Jam”:
IXL was a popular brand of jam when I was a child: you will hear the reason for the name if you spell it out. It seems it was made in a factory in this building on the Hobart docks, now a hotel.