Well, it's just the correct form. Read more about plural endings here: http://mowicpopolsku.com/polish-grammar/cases/nominative/#noun-plural
I guess I will just have to think twice after hearing the spoken version. But frankly, these synthesised voices are really easy to misunderstand. I am hearing 'gdzie są tanych hotele'. Someone needs to interpret the spaces between words better, because even to a native Pole these fragments are very bad to understand. Syllables just blend in a way that is very unnatural.
I think the choice in real conversation might depend to some extent on the degree of certainty. If we're talking about particular hotels that we know to exist, then 'the' is definitely correct; but if we're making a general inquiry about hotels that we think probably exist, then 'there' might feel more appropriate.
okay, so here again we have something that requires specifying. In Polish I an not sure what "gdzie są tanie hotele" means. In English there are two possible questions, a definite and an indefinite question: - Where are the cheap hotels? (The speaker knows or assumes that there are cheap hotels and asks where they are located) - Where are some cheap hotels? (the speaker does not know for sure that cheap hotels exist, but is looking for some and asking where they might be located) - Where are there cheap hotels? (depending on context, the speaker is either responding to another party who just stated that there were cheap hotels and asking them to confirm their location, or alternatively expressing frustration about the lack of cheap hotels by asking a rhetorical question.
In my mind, "gdzie są tanie hotele" is a translation for all 4 - context and tone would have to be known in order to determine which one was the case in point. Since duolingo conveys neither context nor tone, it strikes me that in orde rnot to confuse students, the 3 verbally distinct expressions on English would have to be accepted.
Note: "Where are some cheap hotels?" was rejected as wrong, which I believe is incorrect.