1. Take a 20 minutes' break.

2. Take 20 minutes' break.

3. Take a 20-minute break.

4. That's a 20 minutes' delay.

5. That's 20 minutes' delay.

6. That's a 20-minute delay.

Which of the above sentences is not acceptable?
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Comments  (Page 2) 
However, in quantitative expressions of the following type there is possible variation:

a ten day absence [singular]

a ten-day absence [hyphen + singular]

a ten days absence [plural]

a ten days' absence [genitive plural]

With temporal nouns in the plural, the apostrophe is sometimes omitted: several weeks'/weeks vacation

Pages 325 &1333, A Comprehensive Grammar of the English Language, by Randolph Quirk et al. 1985

Pattern 1 - used adjectivally

We often use a number and a noun combination after a determiner such as an article, this, my etc, and functioning as an adjective before a noun and modifying that noun. In this case, the noun signifying a measure of time, distance, amount, weight etc, is used in the singular:
  • It takes ten minutes to walk there
    It's a ten-minute walk.
  • The holiday is for two weeks
    We're going for a two-week holiday
  • Your hike was fifty miles
    This fifty-mile hiking trip you went on, what was it like?
  • It was five metres to the ground
    It was a five-metre drop to the ground.
  • I've lost five pounds
    Have you seen my five-pound note?
  • He tried to bribe me with a hunded dollars
    He offered me a hundred-dollar bribe
  • The lorry weighs ten tonnes
    It's a ten-tonnne lorry
  • The bottle holds two litres
    It's a two-litre bottle
  • The essay has ten pages
    It's a ten-page essay
Note: the use of hyphens - when used adjectivally like this before another noun, we usually put a hyphen between the number and the first noun, the one indicating the unit of measure. But not all authorities consider this necessary.

Pattern 2 - used as a possessive compound noun

When talking about time or distance, we can also use the number + noun expression as part of a compound noun, in which case they are used in the possessive form with an apostrophe:When there's only one, or the expression ends in a fractionIf the number is one or less, or ends in a fraction (a half, a quarter) etc, the unit of measure is in the singular and is followed by 's.
  • It's only half-an-hour's drive.
  • He was given a week's wages (one week's wages).
  • She's taken a week-and-a-half's break off work
  • It's a mile's walk (one mile's walk) from here.
  • There's half-a-metre's width between the two walls.
  • It was a kilometre-and-a-half's walk to the nearest bus stop.
This construction is also used in some idiomatic expressions:
  • They live very nearby
    They only live a stone's throw away
  • We only just missed it
    We missed by a hair's breadth
Why do we use the possessive form?This is really just another (and more natural) way of saying of.
  • a week's break = a break of a week
  • a mile's walk = a walk of a mile
  • a stone's throw = the throw of a stone
When there's more than oneIf the number is greater than one, the unit of measure is in the plural and is followed by an apostrophe.

Fishing, edited by Horace G Hutchinson, from the "Country Life" Library of Sport. 1904,

  • It's five hours' drive from here.
    (a drive of five hours)
  • We'll be back in two weeks' time.
    (a time of two weeks)
  • It's five miles' walk from here.
    (a walk of five miles)
  • It's ten metres' drop to the ground.
    (a drop of ten metres)
Notice these expressions with a couple, a few etc
  • It's a couple of minutes' walk from the station.
  • There'll probably be a few minutes' wait before he can see us.
Note 1: trip, journey etc - When talking about distance, we only use this construction with words that can mean general distance, such as drive, walk or swim, usually related to verbs. So you can say a place is ten hours' drive from here, but not that it is ten hours' trip from here. With words like trip and journey we'd use the first pattern - we're going on a ten-day trip, it's a four-hour journey, etc.Note 2: the use of apostrophes - A lot of people don't use apostrophes in these expressions. For example, when I googled "and a half week's work", Google asked me 'did you mean "and a half weeks work" '.The apostrophe is stipulated, however, in the style guides of both The Guardian and The Economistnewspapers, and is shown in EFL grammar books such as Raymond Murphy's English Grammar in Use Intermediate (Unit 80 - ten minutes' walk) and Michael Swan's Practical English Usage (p 466 -three miles' walk) as well as many online grammar and writing guides.Which is quite logical if you think about it. The phrase one weeks holiday without an apostrophe doesn't make grammatical sense - it looks as though we have one and the plural weeks. So we really need that apostrophe in this instance - one week's holiday, and it is then only logical to extend it to the plural version as well - three weeks' holiday.

Expressions with money

We really only use money expressions like this with the noun worth (see next section):
  • Can you give me a pound's worth (one pound's worth), please.
  • I bought five pounds' worth of goods.
  • It was a good ten dollars' worth.

A quick note on the noun worth

As a noun, worth is used with expressions of time and money. It's also used in expressions like 'get your money's worth'. To judge by Google Search, it is often used without an apostrophe, but the dictionaries I've checked (both British and American) all show examples with apostrophes, so when you are writing more formally, what usage books call 'careful writing', you'd do better to include the apostrophe:
  • The winner will receive ten pounds' worth of books.
    (Oxford Advanced Learner's)
  • a week's worth of supplies
    (Oxford Online)
  • They've produced five hours' worth of videos
  • The fire caused thousands of pounds' worth of damage.
  • a month's worth of grocery shopping.
  • She has 15 years' worth of experience in advertising.
    (Merriam-Webster Learner's)
  • ten dollars' worth of natural gas
    (The Free Dictionary)
Note - When the amount of money is expressed as a number, we don't use an apostrophe:
  • a chance to win £2000 worth of computing equipment - NOT £2000's worth

When the expression of time, measurement etc is used before an adjective

In the first of each pair, we use the first pattern, the same as before. But in the second example of each pair, the expression of time, measurement etc is being used adverbially, modifying the adjective, and doesn't take an apostrophe.
  • A ten-week-old baby
    The baby is ten weeks old
  • A two-hundred-kilometre-long river
    The river is two hundred kilometres long
Note: pregnant etc - there are one or two exceptions where we use a plural number in the first pattern:
  • The woman is three months pregnant
    She is a three-months-pregnant woman
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