Guangming Primary School planted a total of 21 trees by the side of a road, one tree every 6 meters. How long is the road?
光明小学在一条大路边共栽树21棵，每隔6米栽一棵，问这条路长多少米？
<gadget id="calculator">21 - 1</gadget>
<output>20</output>

<gadget id="calculator">20 * 6</gadget>
<output>120</output>

<result>120</result>