For a batch of parts, Master Zhang completed it in 15 hours alone, and Master Wang completed it in 20 hours alone. If two people do it at the same time, Master Zhang will make 50 more parts than Master Wang when completing the task. How many parts are there in this batch?
一批零件，张师傅独做15小时完成，王师傅独做20小时完成．如果两人同时做，那么完成任务时张师傅比王师傅多做50个零件．这批零件共有多少个？
<gadget id="calculator">60 / 7</gadget>
<output>60/7 = around 8.571429</output>

<gadget id="calculator">(60/7) / 15</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">(60/7) / 20</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">(4/7) - (3/7)</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">50 / (1/7)</gadget>
<output>350</output>

<result>350</result>