Halvfjerds for 70 but yes. Firs is 80 though, so that doesn’t make in much easier.
Fjerde = fourth, fire = four. That makes “half to the fourth” become “halv til fjerde” or “halvfjerds” while “four times twenty” becomes “firsindstyve” and shortened to new Danish “firs”
We don’t really learn the reason, we just memorise the word for the number. Kinda like you know the word “dog” means a four legged cute creature, but not why the name is “dog”. The old rules are not something we are teached, I just got curious after a confused foreigner made me think about the system for a second :p