fizz_buzz.rb

class Fixnum
  def fizz_buzz
    puts 'fizzbuzz' if self % 15 == 0
    puts 'fizz'     if self % 5  == 0
    puts 'buzz'     if self % 3  == 0
    
    puts self
  end
end

Main.rb

require './fizz_buzz'

100.times {|i| (i+1).fizz_buzz}