Plant trees along the perimeter of a circular playground, planting one tree every 4 meters, a total of 30 trees were planted. What is the perimeter of this playground in meters
沿一个圆形操场的周围种树，每隔4米种一棵，一共种了30棵．这个操场的周长是多少米
<gadget id="calculator">30 * 4</gadget>
<output>120</output>

<result>120</result>