If a rectangle is formed by a rope 20 meters long, what is the maximum area of the rectangle?
用20米长的绳子围成一个长方形，则长方形的面积最大是多少平方米．
<gadget id="calculator">20 / 4</gadget>
<output>5</output>

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

<result>25</result>