While learning Danish I figured out that's just the arcane incantation for the number. It's language juju, and you just have to know that it be like it do. Yes, it's syv og halvfems, but the reason behind it doesn't matter anymore. The rest of the double digit numbers are a mess as well; 30 is tredive (three tens in old norse) but starting with 50 it's this weird score (20) and half-to-score system.