The master and apprentice have to make 600 parts, the apprentice makes 15 parts every time, and the master makes 20 parts every time. After the master works for 15 hours, the apprentice will do the rest. How many hours will it take to complete the task?
师徒要做600个零件，徒弟每时做15个零件，师傅每时做20个零件．师傅做15时后，剩下由徒弟做，多少小时才能完成任务？
<gadget id="calculator">20 * 15</gadget>
<output>300</output>

<gadget id="calculator">600 - 300</gadget>
<output>300</output>

<gadget id="calculator">300 / 15</gadget>
<output>20</output>

<result>20</result>