ANSWER:
You might start doing some elaborate fractions but hard riddles like this are sometimes much more about word play than crunching the numbers. Think literally and the answer may just appear right before your eyes.
Because of an 18th Century misunderstanding of Roman numerals. In the “modern” recreation of Roman numerals, the letters IV stand for the number four. The I represents one, the V represents five, and, so it was thought, putting the I before V meant one less than five, or four. Since IV is half the letters of the word FIVE, they represent “half” of FIVE.
It’s a word-play.
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