It takes 12 hours for an apprentice to process a batch of parts. Now there are two masters and apprentices working together to complete the task in 3 hours. It is known that the master processes 15 parts per hour, how many parts are there in this batch?
加工一批零件，徒弟要12小时．现在有师徒二人合作3小时就完成了任务．已知师傅每小时加工15个零件，问这批零件共有多少个？
<gadget id="calculator">1 / 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">(1/3) - (1/12)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">15 / (1/4)</gadget>
<output>60</output>

<result>60</result>