A right-angled trapezoid, if the upper base is extended by 5 cm, the area increases by 25 square centimeters, which is exactly a square. What is the area of the original trapezoid in square decimeters?
一个直角梯形，上底如果延长5厘米，面积增加25平方厘米，这样正好是一个正方形，原来梯形面积是多少平方分米．
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">25 * (2/5) * 25 * (2/5)</gadget>
<output>100</output>

<gadget id="calculator">100 - 25</gadget>
<output>75</output>

<gadget id="calculator">75 / 100</gadget>
<output>3/4 = around 0.75</output>

<result>3/4 = around 0.75</result>