How can you explain that 1999.5 minus 1910 equals SXC in Roman numerals?

Answer 1

Nowadays the equivalent of 1999.5-1910 = 89.5 in Roman numerals would be something like this MCMXCIXS-MCMX = LXXXIXS which doesn't seem to make any logical sense.

Irrespective of today's notation of Roman numerals consider the following:-

MDCCCCLXXXXVIIIIS can be simplified to SMM (1999.5 = 2000-0.5)

MDCCCCX can be simplified to CMMX (1910 = 2010-100)

LXXXVIIIIS can be simplified to SXC (89.5 = 100-10.5)

So it follows that: SMM-CMMX = SXC (1999.5-1900 = 89.5)

Remember that a minus minus is a plus so (-S+MM)-(-C+MM+X) = -S-X+C = SXC

The real rules governing the Roman numeral system were probably changed during the Middle Ages when the Hindu-Arabic numeral system was gradually being introduced into Western Europe presumably to make it easier to convert Roman numerals into Hindu-Arabic numerals and vice versa for trading purposes.

Roman numerals: M=1000, D=500, C=100, L=50, X=10, V=5, I=1 and S=0.5

Source of information: 'History of Mathematics' volume 2 by David Eugene Smith first published in 1925 and ISBN 0486 204 308