There is a rectangular vegetable field, the longer side of which is against the wall, 20 meters long, and it takes 40 meters to enclose this vegetable field with a fence. What is the area of this vegetable field?
有一块长方形菜地，它较长的一条边靠着墙，长20米，用篱笆将这个菜地围起来要40米．这个菜地的面积是多少？
<gadget id="calculator">40 - 20</gadget>
<output>20</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">20 * 10</gadget>
<output>200</output>

<result>200</result>