It takes 5 hours to complete a batch of parts by the master alone; 7 hours to complete the work by the apprentice alone; when the two work together, the master makes 18 more parts than half of the total. How many parts are in this batch?
一批零件，由师傅单独做，需5小时完成；由徒弟单独做，需7小时完成；两人合作，完成任务时师傅做的比总数的一半还多18个．这批零件共多少个？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">(1/5) + (1/7)</gadget>
<output>12/35 = around 0.342857</output>

<gadget id="calculator">1 / (12/35)</gadget>
<output>35/12 = around 2.916667</output>

<gadget id="calculator">(35/12) * (1/5)</gadget>
<output>7/12 = around 0.583333</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(7/12) - (1/2)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">18 / (1/12)</gadget>
<output>216</output>

<result>216</result>