tree is planted every 3 meters between two buildings, and a total of 20 trees are planted. What is the distance between the two buildings?
在两幢楼中间每隔3米种1棵树，共种了20棵，这两幢楼之间距离是多少？
<gadget id="calculator">20 + 1</gadget>
<output>21</output>

<gadget id="calculator">21 * 3</gadget>
<output>63</output>

<result>63</result>