To process a batch of parts, the master can complete it in 9 hours alone, and the apprentice can complete it in 12 hours alone. The master and apprentice worked for 3 hours, and the master made 20 more parts than the apprentice. How many parts are there in this batch?
加工一批零件，师傅独做9小时完成，徒弟独做12小时完成．师徒做了3小时，师傅比徒弟多做了20个，这批零件有多少个？
<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">(1/9) * 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 3</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/3) - (1/4)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">20 / (1/12)</gadget>
<output>240</output>

<result>240</result>