The master and the apprentice make a batch of parts together. The master makes 20 parts per hour, which is twice the number of parts that the apprentice makes per hour. It takes the master and the apprentice 3 hours to complete a batch of tasks. How many parts are there in this batch?
师徒两人共同做一批零件，师傅每小时做20个零件，是徒弟每小时做零件数的2倍，师徒两人共同完成一批任务用了3小时，这批零件有多少个？
<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">20 + 10</gadget>
<output>30</output>

<gadget id="calculator">30 * 3</gadget>
<output>90</output>

<result>90</result>