There are 9 willow trees planted beside a circular pool. If 2 poplar trees are planted between every two willow trees, how many poplar trees should be planted in total?
一个圆形的池边种了9棵柳树，如果在每两棵柳树之间种2棵杨树，一共需要种多少棵杨树．
<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<result>18</result>