Indeed, fyrre (40) is also clearly related to four (or fourth), it only kicks in at 50.
You can recognize the numbers where this system is in place by the ‘s’ at the end, which is a remnant of the “-sindstyve” ending meaning “times twenty”.
If it worked for 30 and 40 they would be “halvandens” and “andens”.
30 (tredive) you cited seems to be the exception, as it’s not “halbtots” or something.
Indeed, fyrre (40) is also clearly related to four (or fourth), it only kicks in at 50.
You can recognize the numbers where this system is in place by the ‘s’ at the end, which is a remnant of the “-sindstyve” ending meaning “times twenty”.
If it worked for 30 and 40 they would be “halvandens” and “andens”.