Yea I already got what you mean. Thanks for the explanation again. Forgive me if you got me wrong :)Part kidding, more pedantic. Maybe useful.
The problem does not say anything about what letter set is expected. It is an assumption made by we as problem solvers that all letters of the alphabet are valid. It's a fair assumption, but an assumption nonetheless.
But the problem does specify a context (license plates) - one in which a non-conventional letter set is specified.
It's the kind of thing that could earn the student bonus marks for pointing out ( definitely in addition to providing the expected answer!)