answer.rb

def get_a_number(line)
  if count = line.instance_variable_get("@count")
    line.instance_variable_set("@count", count += 1)
  else
    line.instance_variable_set("@count", 1)
    count = 1
  end
  line << count
  count
end

def serve_customer(line)
  line.shift
end