It takes 7 hours for the two masters and apprentices to work together to complete a batch of parts. If each person makes one more part per hour, it can be completed 1 hour earlier. How many parts are there in this batch?
师徒二人合作一批零件，要7小时完成，若每人每小时多做1个零件，则可提前1小时完成．这批零件有多少个．
<gadget id="calculator">6 + 6</gadget>
<output>12</output>

<gadget id="calculator">12 * 7</gadget>
<output>84</output>

<result>84</result>