Danish Tens: 20–90
This is where Danish numbers get famously unusual. While 20 and 30 are relatively normal, 40–90 come from an old vigesimal (base-20) counting system — similar to how French uses “quatre-vingts” (4 × 20) for 80.
tyve
twenty
Straightforward — related to Old Norse
tredive
thirty
Straightforward — related to Old Norse
fyrre
forty
Short for "fyrretyve" — 2 × 20
halvtreds
fifty
Half-way to the 3rd score — 2½ × 20
tres
sixty
3 × 20 (three score)
halvfjerds
seventy
Half-way to the 4th score — 3½ × 20
firs
eighty
4 × 20 (four score)
halvfems
ninety
Half-way to the 5th score — 4½ × 20
The Base-20 System
A “score” is 20. The Danish tens from 50 onward are built by counting scores and half-scores:
halvtreds (50)
halvtredje [sindestyve] = half-way to the 3rd score → 2½ × 20 = 50
tres (60)
tre[sindestyve] = 3 × 20 = 60
halvfjerds (70)
halvfjerde [sindestyve] = half-way to the 4th score → 3½ × 20 = 70
firs (80)
fir[sindestyve] = 4 × 20 = 80
halvfems (90)
halvfemte [sindestyve] = half-way to the 5th score → 4½ × 20 = 90
Learning tip
Don't worry about the etymology too much — most Danes don't think about scores when they say these words. Just learn them as vocabulary: halvtreds, tres, halvfjerds, firs, halvfems. The pattern (halv-X-s alternating with X-s) can help you remember the order.