The gardener planted trees along one side of the road (both ends) at an interval of 6 meters. A total of 21 trees were planted. How far is the distance from the first tree to the last tree?
园林工人沿公路的一侧植树(两端都植)，每隔6米植1棵，一共植了21棵，从第一棵到最后一棵的距离有多远？
<gadget id="calculator">21 - 1</gadget>
<output>20</output>

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

<result>120</result>