There is a fence with a length of 20 meters and a rectangular vegetable garden surrounded by the wall on one side. What is the maximum area of the vegetable garden?
有一道栅栏长20米，一边靠墙围成一个长方形的菜园，菜园的面积最大是多少？
<gadget id="calculator">2 + 1 + 1</gadget>
<output>4</output>

<gadget id="calculator">20 / 4</gadget>
<output>5</output>

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

<gadget id="calculator">10 * 5</gadget>
<output>50</output>

<result>50</result>