Plant trees on both sides of an 80-meter-long road. There are 42 trees planted from the beginning to the end, and the distance between two adjacent trees is equal. What is the distance between two adjacent trees?
在一条长80米的公路两旁植树，从起点到终点一共栽了42棵，并且相邻两棵树之间的距离相等，求相邻两棵树之间的距离是多少？
<gadget id="calculator">42 / 2</gadget>
<output>21</output>

<gadget id="calculator">21 - 1</gadget>
<output>20</output>

<gadget id="calculator">80 / 20</gadget>
<output>4</output>

<result>4</result>