number_with_indefinite_article.rb

require 'linguistics'

# Helper function to pick the correct indefinite article for a number
# e.g. number_with_indefinite_article(10)      #=> "a 10"
# e.g. number_with_indefinite_article(80, "$") #=> "an $80"
def number_with_indefinite_article(number, prefix=nil)
  if [11,18].include?(number)
    article = "an"
  elsif number.to_s[0] == "8"
    article = "an"
  else
    word = Linguistics::EN.numwords(number)
    if ["eleven", "eighteen"].include?(word.split(" ")[0])
      article = "an"
    else number.to_s[0] == "1"
      article = "a"
    end
  end
  [article, " ", prefix, number].compact.join
end

I believe only 8, 11, 18 and any subsequent numbers beginning with the number 8, or the words "eleven" or "eighteen" should use "an", so my version just looks for those special cases. the number 1 starts with a "w" sound so it should use "a" rather than "an", which fails the vowel test.