There are 15 willow trees planted around a circular flower bed, and there is a poplar tree between every two willow trees. How many trees are there in total?
一个圆形花坛周围栽了15棵柳树，每两棵柳树之间有一棵杨树，一共有树多少棵．
<gadget id="calculator">15 + 15</gadget>
<output>30</output>

<result>30</result>