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Should I use 'a' or 'an' before numbers? Suppose that we have this sentence:

Their implementation results achieved a (or an?) 85-90% accuracy rate.

6
  • 1
    The choice of "a" or "an", is like no other rule of spelling, because it isn't a rule of spelling. It is a rule of speaking. It is 100% determined by how you say the following word - how you write it is completely irrelevant. (Actually 99.9% because there among the minority of people who use phrases like "an historic occasion" there are a very few who actually pronounce the 'h' in such cases).
    – Colin Fine
    Sep 22, 2016 at 15:08
  • @Colin Fine: I think you're mistaken about an historic. This is one of a very few cases where some extremely carefully-spoken Brits make a specific point of aspirating the /h/ after an. Since I wrote this comment several years ago it's become pretty clear to me that practically every time I hear it (which turns out to be a lot, since I've developed "blue car syndrome" regarding the usage), it's from someone I would call a careful speaker. Sep 22, 2016 at 15:45
  • And these "extremely carefully-spoken Brits" amount to something other than a very few among a minority, @FumbleFingers? ;-)
    – Colin Fine
    Sep 22, 2016 at 17:32
  • 1
    @Colin Fine: Well, it's hard to say (mainly because of "blue car syndrome"). Obviously there are four theoretically possible permutations, within which I'd exclude a 'istoric as effectively "unpronounceable" anyway. But my gut feeling is that whereas a historic is unquestionably the most common of the other three, I think an historic is actually more common than an 'istoric (at least, if we restrict ourselves to apparently reasonably competent native speakers on the BBC, or giving lectures on Youtube, etc.). Sep 22, 2016 at 18:03
  • Maybe. But if I encounter "an historic" and read it normally, I won't sound the 'h'.
    – Colin Fine
    Sep 22, 2016 at 18:17

2 Answers 2

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It depends how you would read it;

... achieved an eighty-five to ninety percent ...

So "an" in this case. But

... achieved a seventy-five to eighty percent ...

"a" because seventy doesn't begin with a vowel (when you speak it).

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It is based on how it sounds. Example:

a 1    a 2    a 3    a 4    a 5    
a 6    a 7    an 8   a 9    a 10

an 11  a 12   a 13   a 14   a 15
a 16   a 17   an 18  a 19   a 20

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