I had to look this up. What the fuck? They came up with numbers up to 60 and then just said “eh, fuck it” and made 70 “sixty-ten”, 80 “four-twenties” and 90 “four twenties ten”.

The other way around. We started with base 20 everywhere then simplified some of it.

During medieval times it used to be :
10 Dix (10)
20 Vingt (20)
30 Vingt et dix (20+10)
40 Deux-vingt (2x20)
50 Deux-vingt et dix (2x20+10)
60 Trois-vingt (3x20)
70 Trois-vingt et dix (3x20+10)
80 Quatre-vingt (4x20)
90 Quatre-vingt et dix (4x20+10)

Then they switched to base 10... But only up to 70 for some reasons in France. Belgium and Switzerland (and some parts of France) have gone all the way to 100 by using Septante (70), Octante or Huitante (80) and Nonante (90).