I think it's basically a fragment in either language. It's not really a concept without some context. But if the fiftieth is one half, then the whole is the hundredth. So we have a goal of 100 somethings. Maybe we want to live a hundred years, and today is our 50th birthday, so with regard to the goal we say: the fiftieth is one half.
I know some people don't find contextualizing the sentences useful and just advocate sticking to the grammar and vocab, but I like to think about the sentences as I go along myself.