A pedestrian street is 800 meters long. If a street lamp is installed every 40 meters on both sides of the street (also installed at both ends), how many street lamps need to be installed in total?
一条步行街长800米．如果在街的两侧每隔40米安装一盏路灯(两端也要安装)，一共需要安装多少盏路灯？
<gadget id="calculator">800 / 40</gadget>
<output>20</output>

<gadget id="calculator">20 + 1</gadget>
<output>21</output>

<gadget id="calculator">21 * 2</gadget>
<output>42</output>

<result>42</result>