Where does “trettio” come from?

trettio (Swedish) comes from Old Swedish þrætighi, from Proto-Germanic þrīz tigiwiz — thirty.

trettio (Swedish): thirty

Definitions

Step	Language	Word	Meaning
1	Old Swedish	þrætighi	thirty
2	Proto-Germanic	þrīz tigiwiz	thirty

trettio — Etymology, Origin & Word History | Etymology Explorer