The uncle of the duck farm is going to use a fence on one side of the river bank to enclose a rectangular duck farm with a length of 20 meters and a width of 10 meters. In order to save materials, which side should he put on the river bank? What is the perimeter of the fence at this time?
养鸭场的叔叔准备用篱笆靠一边河岸围一个长20米，宽10米的长方形鸭场，为了节省材料，他应该把哪边靠着河岸？这时篱笆的周长是多少米？
<gadget id="calculator">10 * 2</gadget>
<output>20</output>

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

<result>40</result>