Concerning 40 - “sorok” was a name for a unit of measure for fur. One sorok was a bundle consisting of 40 animal furs. As for 90 - devyanosto - I think it come

Concerning 40 - “sorok” was a name for a unit of measure for fur. One sorok was a bundle consisting of 40 animal furs. As for 90 - devyanosto - I think it comes from “9 to 100”. Its strange though its 9 and not 10.