The Danish numbering system for multiples of ten from 50 to 90 is intricate. “Halvtreds” for fifty is a combination of “halvtredje” (two and a half) and “sindstyve” (times 20), yielding 50.
Complexity deepens:
Sixty, “tres,” is “tre” (three) and “sindstyve” (3 times 20).
Seventy, “halvfjerds,” is “halvfjerde” (4-½) and “sindstyve” (3½ times 20).
Eighty, “firs,” is “fire” (four) and “sindstyve” (4 times 20).
Ninety, “halvfems,” is “halvfemte” (5-½) and “sindstyve” (4½ times 20).
Historically, Danish used “siutyugh” (six tens) until around 1300. A text found in the city of Flensborg from that era introduces multiples of 20. The vigesimal system likely started in Western Denmark and spread eastward.
Similarities exist in French, where “quatre-vingt” (80) and “quatre-vingt-dix” (90) are based on twenty. Vigesimal counting appears in various Indo-European languages, possibly influenced by pre-Celtic languages.
More number facts.