To process a batch of parts, the master alone needs 10 hours to complete the work, and the apprentice needs 15 hours to complete the work alone. The master processes 30 more parts per hour than the apprentice. How many parts are there in this batch?
加工一批零件，师傅单独做需要10小时完工，徒弟单独做需要15小时完工，师傅每小时比徒弟多加工30个．这批零件有多少个？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">(1/10) - (1/15)</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">30 / (1/30)</gadget>
<output>900</output>

<result>900</result>