Question

What's wrong with this sentence?

Un "A" est meilleure q'un "F".

I think there's something wrong with q'un

Answer 1

The letters of the alphabet are masculine. Look at the adjective "meilleure" as that is feminine. You need "meilleur."

Sra (aka Mme)

Answer 2

If you are not allowed to change the sentence, then the problem is what you stated:
qu'un has been misspelled as q'un
and
meilleure should be written meilleure.

I would prefer to say:
Une note "A" est meilleure qu'une note "F".

Here meilleure is correct because "une note" is feminine.

Answer 3

Oops, sorry Angie. There was a typo in my answer, but Sra has given the correct answer. Hope that didn't confuse you.

The corrected line should read:
"meilleure should be written meilleur."