I think in the following sentence, alle should be replaced with jede:
Diese Busse fahren alle fünfzehn Minuten vom Hauptbahnhof ab.
It means that the buses leave every fifteen minutes, doesn't it? Are both correct? Do they mean exactly the same?
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You use alle for "15 minutes" because "minutes" is plural. You would use jede for singulars, like jede halbe Stunde. You could not use them vice-versa.
I think it is something like a fixed term.
alle bezeichnet den wiederkehrenden Wechsel; immer wieder nach alle halbe(n), alle (drei) Stunden mitteldeutsch aller aller fünf Minuten, zwei Jahre, drei Schritt