Usage Note
Demi is invariable when it precedes a noun and is joined by a hyphen: une demi-heure (half an hour), un demi-litre (half a litre). When it follows the noun it agrees in gender: une heure et demie (an hour and a half). As a standalone noun, un demi is a half-pint of beer.
Examples
"Je reviens dans une demi-heure."
Natural Translation
I'll be back in half an hour.

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