Use singular when it is functioning as an adjective; use plural when it is functioning as a noun: This is a three-student project. the three-person apartment It's a two-bedroom apartment. This project requires 3 students.
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LissionI sometimes see 'two-roomed apartment', as in sentence 3. Is it correct?I have never seen this usage. I guess it's possible that it's used in some specialized context but not in everyday speech.