generally, 7 and 13 get a lot of attention because a)they are prime b)1/7 and 1/13 have long period when written as decimal fraction - so you keep stumbling in your calculations every time you encounter them

1/7 in base 20 takes surprisingly short form of 0.(1h) (h is 17), unlike 1/9 or 1/11 - so I wonder if there's Mayan prejudice on those instead