Dig a canal 1,200 meters long, and complete 420 meters in 7 days. According to this calculation, how many days will it take to complete this task? (Answer with proportional knowledge)
挖一条水渠长1200米，7天完成420米，照这样计算，完成这一项任务要多少天？(用比例知识解答)
<gadget id="calculator">1_200 * 7</gadget>
<output>8_400</output>

<gadget id="calculator">8_400 / 420</gadget>
<output>20</output>

<result>20</result>