Let's look at 60 (and 80): Tres (60) comes from tresindtyve (tre-sinde-tyve), which means 3 times 20
The same goes for 80: fir(e)sindetyve (4 times 20) = shortened to firs. Sinde is an old word (now obsolete) meaning times
Now for the hard part: 50, 70, 90 Halv 3 = 2½, halv 4 = 3½, halv 5 = 4½ Halv 3 x 20 = halvtredje-sinde-tyve - shortened to halvtreds (50). Halv 4 x 20 = halvfjerde-sinde-tyve - shortened to halvfjerds (70). Halv 5 x 20 = halvfem-sinde-tyve - shortened to halvfems (90).