A rectangular vegetable field is 20 meters long and 10 meters wide. If one long side is against a wall and the other three sides are not fenced, what is the minimum length of the fence needed?
一个长方形菜地，长20米，宽10米，如果一条长边靠墙，另外三边未上篱笆，至少需要多少米长的篱笆？
<gadget id="calculator">10 * 2</gadget>
<output>20</output>

<gadget id="calculator">20 + 20</gadget>
<output>40</output>

<result>40</result>