Plant trees along the perimeter of a rectangular playground, one tree every 4 meters, and a total of 30 trees are planted. What is the perimeter of the playground in meters?
沿一个长方形操场四周种树，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>