What is the maximum area of a rectangle or square surrounded by a wire 20 meters long?
把一根长20米的铁丝围成一个长方形或正方形，面积最大是多少平方米．
<gadget id="calculator">20 / 4</gadget>
<output>5</output>

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

<result>25</result>