The following sentence from a course quiz of mine deals with the adjectival noun "der/die Kranke" (sick person):
Alle [...] (Krank-) brauchen Ruhe.
Here is how I approached this:
There is no article, meaning the adjectival noun must assume a strong ending.
The case here is the nominative plural.
Therefore, since the strong ending for nominative plural is -e, I answered:
Alle Kranke brauchen Ruhe.
However, my instructor says this needs to be:
Alle Kranken brauchen Ruhe.
Could someone please help me understand why this noun is declined like this? I thought perhaps der Kranke is N-declined, but that doesn't seem to be the case from my research.
The only other explanation I can come up with is that this is a mixed ending and not strong, but "alle" is not an indefinite article like "ein/kein/mein/sein/etc."