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 were repaired in 2 days. According to this calculation, how many days can it be completed in advance?
修一条水渠，计划每天修300米，40天完成．实际上2天就修了800米，照这样计算，可以提前多少天完成？
<gadget id="calculator">800 / 2</gadget>
<output>400</output>

<gadget id="calculator">40 / 400</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">300 * (1/10)</gadget>
<output>30</output>

<gadget id="calculator">40 - 30</gadget>
<output>10</output>

<result>10</result>