“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?

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    It might have been a mockery of the old english system ( projectbritain.com/moneyold.htm ) the wizarding world does seem to be behind in a lot of ways (quills/owls/latin/etc) though that is just speculation on my part. as for primes they are the most likely numbers to be picked when someone is asked at random, it is possible she just didn't think out the implications. – Ummdustry Jan 11 '18 at 20:34