In order to green the road, the Garden Bureau planted a tree every 4 meters from the beginning to the end of a section of the road. A total of 21 trees were planted. Now it is necessary to plant a tree every 5 meters from the beginning to the end. How many trees will be planted in total?
园林局为了绿化公路，在一段公路的一边从头到尾每隔4米栽一棵树，一共栽了21棵树．现在要改成从头到尾每隔5米栽一棵树，一共要栽多少棵树？
<gadget id="calculator">21 - 1</gadget>
<output>20</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">20 * (4/5)</gadget>
<output>16</output>

<gadget id="calculator">16 + 1</gadget>
<output>17</output>

<result>17</result>