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Today---- of the Earth live on a very small percentage of the Earth's land surface.
A, about two-thirds populated
B, the population is about two-thirds
C, about two-thirds of the population
D, of about two-thirds the population is

We all know that the answer is C. But what I wanna ask here is the word 'two-thrids'.

Before answering that question, I thought 'two-thirds' was wrong, not a word exactly. I mean, we can say two hours or two-hour, but we absolutely cannot say 'two-hours', right? So, I thought two-thirds was wrong too. hehe.
So, I want to know why it works.

And two-thirds seems to be used as a noun here which represents 'the population'. Can we use it as an adj to modify 'population too', say, two-thirds population?
Comments  
It is a fraction. The hyphen is incorrect (?) or at least unnecessary (I suppose it is a remnant of the slash in '2/3' in the mind of the writer) Better would be 'about two thirds of the population'.

As an adjective, it should hyphenated-- 'a two-thirds majority of the Senate is required', 'a three-quarter length dress'; it is rare in this usage, though.

It doesn't work for X'two-thirds population'-- with fractions less than 1, we usually use 'of a + singular noun' (vide Swan), hence 'thirds' remains a noun.

It doesn't work for X'two-thirds population'-- with fractions less than 1, we usually use 'of a + singular noun' (vide Swan), hence 'thirds' remains a noun.


I am not sure if I get this one.

So, because two-thirds is less than 1, 'two-thrids population' is wrong? I have no clue, why?
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'With fractions less than 1, we usually use 'of a + singular noun' (vide Practical English Usage by Michael Swan)'

Usage, Jeff; I have no other explanation to offer. Perhaps another mod will pop by with a bright idea.
I see Mr Micawber.

So, we can say one and two-thirds + noun (p), but we absolutely cannot say two-thirds + noun. We have to say two-thirds of a + noun (s), since two-thirds is less than 1, right?

Another one I am confused about is 'hyphen'.
It seems two-thirdS is correct, what about two-hours or two-years or two thirds?
Can you tell me how many ways to express a fraction, say 2/3? Two out of three? Two thrids? Two-thirds?

Thankkk Youuu
'Absolutely cannot' is an unusual phrase in discussing English grammar. Let me say that I 'ate one and two-thirds apples and three-fifths of an apricot', and that would be reasonably acceptable.

As I said, the hyphen is unnecessary if not wrong, but is in popular use for fractions (which does not include 'two hours' or 'two years'). I would suggest that the hyphen be used for adjectives only:

I ate two thirds of the kumquat.
The kumquat is two-thirds eaten.
The meeting lasted two hours.
It was a two-hour meeting.

'2/3' I would read as 'two thirds' only, I am sorry to say. '3/4' however can be 'three fourths' or 'three quarters'.
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Okey, I will bear them in mind. Thank you Mr Micawber.