Master and apprentice work together to process 840 parts. If the master does it for 9 days first, and then the apprentice does it for 2 days, then the task can be completed; How many parts do you ask your apprentice to process every day?
师徒共同加工840个零件，师傅先做9天，再由徒弟做2天，则可完成任务；如果徒弟先做6天，师傅再做6天，也能完成任务．求徒弟每天加工多少个零件？
<gadget id="calculator">840 / 6</gadget>
<output>140</output>

<gadget id="calculator">140 * 2</gadget>
<output>280</output>

<gadget id="calculator">840 - 280</gadget>
<output>560</output>

<gadget id="calculator">9 - 2</gadget>
<output>7</output>

<gadget id="calculator">560 / 7</gadget>
<output>80</output>

<gadget id="calculator">140 - 80</gadget>
<output>60</output>

<result>60</result>