An electrical appliance factory originally planned to produce 840 refrigerators in 15 days. After technological innovation, it actually produced 420 refrigerators in 5 days. According to this calculation, how many days can the task be completed in advance?
某电器厂原计划15天生产电冰箱840台，技术革新后，实际5天就生产了420台，照这样计算，可以提前几天完成任务？
<gadget id="calculator">420 / 5</gadget>
<output>84</output>

<gadget id="calculator">840 / 84</gadget>
<output>10</output>

<gadget id="calculator">15 - 10</gadget>
<output>5</output>

<result>5</result>