The system is actually based on scores, 20, which is called a snes in older Danish, so halvtreds is short for halv tredje snes, the half third score, and 60 is tres, short for tre snese, i.e. three scores and so on. So for the tens between 50 and 90, we count scores, and if it's not a whole number of scores, we name it the half of the score that we are into. It's also preserved in a very infrequently used variant word for 80, firsindstyve, which is just 4 score, more explicitly (tyve is the modern word for twenty). In conclusion: Yes, the Danish number system is relatively silly.