If a tree is planted every 4 meters around the playground, and a total of 21 trees are planted, what is the perimeter of the playground in meters?
在操场四周每隔4米植一棵树，共植21棵，则操场周长多少米．
<gadget id="calculator">21 * 4</gadget>
<output>84</output>

<result>84</result>