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If you have only 5 hours of sleep for three nights, you don't need to sleep an extra 9 hours on the weekend.

I can't understand why the phrase "an extra 9 hours" is plural form as using 'hours'.

  • Here is a discussion that doesn't exactly answer your question, but it may help understanding when to use hour and when to use hours: Should you say hour or hours in this type of sentence? – ColleenV parted ways May 5 '17 at 3:08
  • You can rephrase: ...you don't need to sleep for 9 hours more on the weekend or ...you don't need additional 9 hours of sleep on the weekend – SovereignSun May 5 '17 at 6:01
  • Is the issue the use of "an", which refers to a single item, vs. the plural "hours"? The link in Cookie Monster's answer covers it well. The gist is that you are talking about a single period (the reason for "an"), and 9 hours describes the period. The "an" refers to the period, not the 9 hours. – fixer1234 May 5 '17 at 6:29
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I can't explain that either, but that's just how it works in English. If you've got an amount of something and you precede it with an adjective, you must place an indefinite article in front of the whole expression.

Example:

a whooping 20 dollars

For more information, check this post: a joyous and restful three days — a three days?

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