Can we say a half hour in place of half an hour?
If no , then what is difference between two?
The show lasted a half hour / half an hour.
As an attributive adjective, however, only the first will do:
It is a half-hour show.
Use perhaps has to do with stress and ease of articulation in context:
I'll meet you in half an hour.
A half hour is not long enough for what I have in mind.
People are waiting to help.
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