A workshop processes a batch of parts. If it processes 20 parts per day, it can be completed in 15 days. In fact, 100 parts are added in 4 days. According to this calculation, how many days will it take to complete the processing task?
某车间加工一批零件，如果每天加工20个，15天可以完成，实际4天就加个了100个，照这样计算，再需几天就可以完成加工任务？
<gadget id="calculator">100 / 4</gadget>
<output>25</output>

<gadget id="calculator">15 / 25</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">20 * (3/5)</gadget>
<output>12</output>

<gadget id="calculator">12 - 4</gadget>
<output>8</output>

<result>8</result>