[S]he found the old Imperial system to be much more picturesque and quirky and therefore more appropriate for the society she was creating.
You may notice on the homepage of the Lexicon that it always shows the current exchange rate between a galleon and a range of different worldwide currencies. But did you ever wonder how J.K. Rowling came up with this seemingly confusing system of wizard money? Hagrid explains how it works in the first book, telling Harry that there are 29 knuts to a sickle, 17 sickles to a galleon, and that “it’s easy enough” to work out. (PS5)
There are a couple of possibilities. J.K. Rowling has confessed many times that math is not a strength of hers (TLC, JKR). If Muggle math is confusing to her, perhaps she developed this confusing system of wizard currency to illustrate that feeling of being confused by something other people seem to find ‘easy enough.’ However, Jo has also said that keeping the Imperial system in the book was a deliberate decision, even though the editor wanted to change all the weights and measures to metric (Pm), and that was because she found the old Imperial system to be much more picturesque and quirky and therefore more appropriate for the society she was creating. So it’s likely that she was influenced by the old English system of money where there were 12 pennies in a shilling and 20 shillings to a pound.
Still, 12 and 20 are friendlier numbers to work with than 17 and 29, which you’ll probably recognise are both prime numbers. Prime numbers are thought to have mystical properties, making them an appropriate choice for the wizarding currency, and other prime numbers pop up throughout the books too. More on those in a future episode. So perhaps J.K. Rowling is mathematically challenged in some areas, but she’s certainly no stranger to the magical properties of numbers.