A batch of parts, A alone takes 15 hours to complete, B makes 25 parts per hour, and two people work together to complete it in 6 hours. How many parts are there in this batch?
一批零件，甲单独做要15小时完成，乙每小时做25个零件，两人合做6小时完成．这批零件有多少个？
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

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

<gadget id="calculator">(1/6) - (1/15)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">25 / (1/10)</gadget>
<output>250</output>

<result>250</result>