A total of 21 trees are planted from one end to the other end of a path. If every two trees are spaced 5 meters apart, how long is the path in meters?
一条小路从一端到另一端一共栽了21棵树．如果每两棵树都间隔5米，这条小路长多少米？
<gadget id="calculator">21 - 1</gadget>
<output>20</output>

<gadget id="calculator">5 * 20</gadget>
<output>100</output>

<result>100</result>