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Which of the following is correct, and why?

It cost us a tenth what was spent previously.

or

It cost us a tenth of what was spent previously.

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    Well, which of these does not even make sense? – RegDwigнt Jun 2 '15 at 14:52
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    When talking about fractions, I think the only place you can get away with dropping the of is half. Others pretty much need the of. – Tushar Raj Jun 2 '15 at 14:53
  • The preposition of is required. It may sometimes be omitted in informal speech, but it's not grammatical to do so. – Kris Jun 2 '15 at 14:54
  • Over 100 000 Google hits for 'a tenth the' (raw data). For "a tenth what", the early results are false positives. Not all apparently logical deletions are idiomatic; English is never claimed to follow precise logic. – Edwin Ashworth Jun 2 '15 at 15:26
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This is one of those relatively uncommon cases where usage has become more restrictive over time...

Personally, I don't see anything wrong with not including the preposition, but obviously over the past century increasing numbers of speakers/writers have come to prefer that it be used.

I'm not sure it's meaningful to ask why people prefer the preposition today. For the first 100 years in the above chart they clearly didn't have any such preference, and it's not obvious to me there's been any more general shift in how prepositions are used in English since then. It's probably just a random change in idiomatic preference, that gained traction for no particular reason.


EDIT: Noting @Tushar Raj's comment above, and this NGram for a third rather than a half, I should acknowledge that of has always been far more likely with fractions other than half. So arguably the trend representd by my graph above is simply "normalization" of an "irregular" usage.

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    So about 1845, the (estimated) popularity of 'only half of what' was only half what that of 'only half of what' was, whereas about 1970, the (estimated) popularity of 'only half what' was only half of what that of 'only half what' was. – Edwin Ashworth Jun 2 '15 at 15:31
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    @Edwin: I might of said that myself - but I didn't, because I thought people might of thought I'd used of inappropriately! :) – FumbleFingers Jun 2 '15 at 15:40
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The latter is correct.

cost us a tenth of what was spent previously

If you are describing an amount, or portion, you always have to use OF to connote what it is you are taking a portion of.

Only if you were describing multiples of an object could you leave out 'of' (ie. A third of an apple / 6 apples)

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    Not only can you leave of out when dealing with multiples, you must leave it out. *6 of apples is incorrect. – Mr. Shiny and New 安宇 Jun 2 '15 at 15:25
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    You offer no evidence for this statement, and it is not entirely accurate. There are over 100 000 Google hits for 'a tenth the' (raw data). 'in a tenth the time' is a common deletion in spoken English. An example from the internet: 'the best programmers can solve a given problem in a tenth the time'. – Edwin Ashworth Jun 2 '15 at 15:35

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