For a batch of parts, Master Zhang alone completed 20 hours, and Master Li completed 30 hours alone, and Master Zhang made 20 more parts per hour than Master Li. How many parts are there in this batch?
一批零件，张师傅单独做20小时完成，李师傅单独做30小时完成，而且张师傅比李师傅每小时多做20个．这批零件共有多少个？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) - (1/30)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">20 / (1/60)</gadget>
<output>1_200</output>

<result>1_200</result>