We use base-10 mathematics because we have ten fingers,
My ancestors were astucious enough to know that they have also 10 toes and they used a base 20 system that survived a little in my language and in the collective culture.
For exemple, there is a hospital in Paris called les quinze-vingts
(the 15-20) because it hosted 300 patients when it was created during the Middle Ages.
Another exemple : 80 = quatre-vingt = 4-20 litterally.