Reminded me of the example in "Vedic mathematics" - the "ekadhikena purvena" ("one more than the previous one") sutra using which 1/19 was expanded into decimals. For 19, "one more than the previous one" is 1+1=2 so doubling and adding becomes the rule. For 29, it is 2+1=3, so tripling and adding.

https://en.wikibooks.org/wiki/Vedic_Mathematics/Sutras/Ekadh...