resp.rb

def average_response_time(enquiries)
  total_response_time = enquiries.inject(0) do |time, enquiry|
    if enquiry.emails.size > 0
      first_response = enquiry.emails.order('created_at DESC').first
      time += first_response.created_at - enquiry.created_at  
    end    
  end 
  total_response_time / enquiries.count
end