[identity profile] maharetr.livejournal.com 2009-07-09 11:29 am (UTC)(link)
*hits Google, because I can't remember either*

It depends on the sentence, but it sounds like it's either 'fifty-first century' or 'fifty-first-century' which... the second one looks weird. Whatever. Cite here (http://www.getintouni.com/Free/GrammarStyle/GrammarC/). :)