Huh? 50 is halvtreds (20 * 2.5)

70 is actually halvfjerdensindstyve, literally "half to the four(th20) times 20". So the base number (20) * 3.5...

The problem is, Danish people can't understand each other even with the simplest of words, so they just say halvfjerds.

70 = halfway to the fourth twenty

In other words, three twenties are 60, four twenties are 80, halfway between them is 70.

Same for the other ones: 50 is halfway to the third twenty, 90 is half more than the fourth twenty.

Oh yea, "half to five" aka 4.5 times 20. Very intuitive...