Dr Collins Empirical Formula question

Didn't find a thread for this exact problem. And hopefully I am placing this thread in right area. Anyway, the question asks for empirical formulas. One of which is ammonium nitrite (NH4NO2). The answer stated by the review is NH2O which doesn't make sense to me since I am assuming this is an ionic bond between two polyatomic ions. Another example was sodium sulfate (Na2SO4) where the answer was Na2SO4. How come for ammonium nitrite it doesn't treat each of the two polyatomic ions as a... Dr Collins Empirical Formula question
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