Plant a willow tree every 5 meters around a circular pond, exactly 30 trees are planted, what is the circumference of the pond in meters?
在一个圆形池塘周围每隔5米植一棵柳树，正好植树30棵，这个池塘的周长是多少米．
<gadget id="calculator">5 * 30</gadget>
<output>150</output>

<result>150</result>