Is the hover text right? Shouldn't it be base 21 with your toes included?
In the same way that just one hand would be base 6. But two hands doesn't make it base 12, it's only base 11 because you don't get two lots of 0. So adding your feet is only base 21.
By the way, if you ever really need the alt text on mobile, go to the website, add a .m subdomain (so it's m.xkcd.com) and click the alt text button below the comic.