A rectangular vegetable field is surrounded by a 20-meter-long bamboo fence. What is the maximum area of the vegetable field in square meters?
用20米长的竹篱笆围出一块长方形菜地，菜地面积最大是多少平方米．
<gadget id="calculator">4 * 6</gadget>
<output>24</output>

<result>24</result>