Where does “vīftien” come from?

vīftien (Old Dutch) comes from Proto-Germanic fimftehun, from Proto-Germanic tehun, from Proto-Indo-European déḱm̥t — ten.

Ancestry of “vīftien”, step by step

StepLanguageWordMeaning
1Proto-Germanicfimftehunfifteen
2Proto-Germanictehunten
3Proto-Indo-Europeandéḱm̥tten
Every word from Proto-Indo-European déḱm̥t →Every word from Proto-Germanic tehun →Every word from Proto-Germanic fimftehun →