The master and the apprentice make a batch of parts together, the master makes 25 parts per hour, and the apprentice makes 20 parts per hour. When the master completes half of the task, the apprentice still has 30 parts to complete. How many parts are there in this batch?
师徒俩合做一批零件，师傅每小时做25个，徒弟每小时做20个．当师傅完成任务的一半时，徒弟还有30个没有做，这批零件共有多少个？
<gadget id="calculator">25 - 20</gadget>
<output>5</output>

<gadget id="calculator">30 / 5</gadget>
<output>6</output>

<gadget id="calculator">6 * 25 * 2</gadget>
<output>300</output>

<result>300</result>