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.
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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).
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