To dig a canal, if you dig 500 meters in 4 days, it will take 40 days, if you dig 800 meters in 4 days, how many days will it take to dig? (Positive and inverse proportional solutions).
挖一条水渠，如果4天挖500米，要40天，如果4天挖800米，照这样计算要挖多少天？(正、反比例两种方法解)．
<gadget id="calculator">500 / 4</gadget>
<output>125</output>

<gadget id="calculator">125 * 40</gadget>
<output>5_000</output>

<gadget id="calculator">800 / 4</gadget>
<output>200</output>

<gadget id="calculator">5_000 / 200</gadget>
<output>25</output>

<result>25</result>