The master and the apprentice process a batch of parts together. The master processes 9 parts per hour, and the apprentice processes 5 parts per hour. When the task is completed, the apprentice processes 120 fewer parts than the master. How many parts are there in this batch?
师徒两人共同加工一批零件，师傅每小时加工9个，徒弟每小时加工5个，完成任务时，徒弟比师傅少加工120个．这批零件共有多少个？
<gadget id="calculator">9 - 5</gadget>
<output>4</output>

<gadget id="calculator">120 / 4</gadget>
<output>30</output>

<gadget id="calculator">9 + 5</gadget>
<output>14</output>

<gadget id="calculator">30 * 14</gadget>
<output>420</output>

<result>420</result>