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“The gold ones are Galleons,” he explained. “Seventeen silver Sickles to a Galleon and twenty-nine Knuts to a Sickle, it’s easy enough.”
Harry Potter and the Philosophers Stone – Chapter 5 – Diagon Ally
The fractions 17, and 29 are not only odd (unusual, bizarre, eccentric), and odd (uneven), they are primes. Clearly this is not a well thought out system, and thus produces a really humorous effect on the text.
I initially thought that this is a diss on the imperial measurement system, because I was under the (wrong) impression that Hagrid had a southern US accent.
Has JKR ever said why she chose these fractions for the wizarding currency subunits?