A worker processes a batch of parts, and plans to process 20 parts per hour, which can be completed in 12 hours. Now it needs to be completed 2 hours ahead of schedule, how many extra parts are processed per hour on average?
一个工人加工一批零件，计划每小时加工20个，12小时可以完成．现在要提前2小时完成，平均每小时要多加工多少个零件？
<gadget id="calculator">20 * 12</gadget>
<output>240</output>

<gadget id="calculator">12 - 2</gadget>
<output>10</output>

<gadget id="calculator">240 / 10</gadget>
<output>24</output>

<gadget id="calculator">24 - 20</gadget>
<output>4</output>

<result>4</result>