If the length of a rectangular piece of land is reduced by 400 meters, the area is reduced by 20 hectares, and the remaining part is exactly a square. What is the area of the original rectangular land in hectares?
一块长方形土地的长减少400米，面积就减少20公顷，这时剩下的部分正好是一个正方形．原来长方形土地的面积是多少公顷？
<gadget id="calculator">1 * 10_000</gadget>
<output>10_000</output>

<gadget id="calculator">10_000 / 400</gadget>
<output>25</output>

<gadget id="calculator">20 * 25</gadget>
<output>500</output>

<gadget id="calculator">500 + 400</gadget>
<output>900</output>

<gadget id="calculator">10_000 / 400 / 10_000</gadget>
<output>1/400 = around 0.0025</output>

<gadget id="calculator">900 * 20 * (1/400)</gadget>
<output>45</output>

<result>45</result>