The two masters and apprentices work together to make a batch of parts, and it takes 7 hours to complete. If each person makes one more part per hour, it can be completed 1 hour earlier. How many parts are there in this batch?
师徒二人合做一批零件，要7小时完成，若每人每小时多做一个零件，则可提前1小时完成．这批零件有多少个．
<gadget id="calculator">1 * 2</gadget>
<output>2</output>

<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

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

<gadget id="calculator">(1/6) - (1/7)</gadget>
<output>1/42 = around 0.02381</output>

<gadget id="calculator">2 / (1/42)</gadget>
<output>84</output>

<result>84</result>