To build a canal, it is planned to build 300 meters per day, and it will be completed in 40 days. In fact, 800 meters will be repaired in 2 days. According to this, how many days can it actually be completed? (Use positive and negative ratios to solve)
修一条水渠，计划每天修300米，40天完成，实际2天修了800米，照这样，实际可用多少天完成？(用正、反两种比例解)
<gadget id="calculator">300 * 40</gadget>
<output>12_000</output>

<gadget id="calculator">800 / 2</gadget>
<output>400</output>

<gadget id="calculator">12_000 / 400</gadget>
<output>30</output>

<result>30</result>