80 trees are planted on both sides of a road, and the distance between each two trees is 4m. What is the length of this road in m?
在一段公路两旁栽80棵树，两头都栽，每两棵之间的距离是4m，这段公路长多少m．
<gadget id="calculator">80 / 2</gadget>
<output>40</output>

<gadget id="calculator">40 - 1</gadget>
<output>39</output>

<gadget id="calculator">39 * 4</gadget>
<output>156</output>

<result>156</result>