If the upper base of a right-angled trapezoid is extended by 4 centimeters, its area will increase by 20 square centimeters and become a square. What is the area of the original trapezoid?
将一个直角梯形的上底延长4厘米，它的面积就比原来增加20平方厘米，且成了一个正方形．原来梯形的面积是多少？
<gadget id="calculator">2 / 4</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">20 * (1/2)</gadget>
<output>10</output>

<gadget id="calculator">10 * 10</gadget>
<output>100</output>

<gadget id="calculator">100 - 20</gadget>
<output>80</output>

<result>80</result>