address.rb

class Address
  attr_reader :city, :state

  def initialize(city, state)
    @city, @state = city, state
  end

  def ==(other_address)
    city == other_address.city && state == other_address.state
  end
end

based on http://www.informit.com/articles/article.aspx?p=2220311&seqNum=11