If the perimeter of a rectangle is 20 centimeters, what is the maximum area in square centimetres?
一个长方形的周长是20厘米，它的面积最大是多少平方厘米．
<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 / 2</gadget>
<output>5</output>

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

<result>25</result>