Hi @Florian23232
If both questions returns a number, then you should be able to do something like this:
select cast({{#123}} as float) / cast({{#456}} as float) where 123 and 456 are the question IDs.
There's other ways of using saved questions:
https://www.metabase.com/docs/latest/questions/native-editor/referencing-saved-questions-in-queries
And depending on how your data is stored, then you might be able to use Share expression function, but that's hard to tell with the limited information.
https://www.metabase.com/docs/latest/questions/query-builder/expressions