Install street lamps on both sides of a road with a total length of 3 kilometers (no installation at both ends), and install one every 20 meters. How many lamps should be installed in total?
在一条全长3千米的公路两旁装路灯(两端都不安装)，每隔20米装一座，一共要装多少座？
<gadget id="calculator">3 * 1_000</gadget>
<output>3_000</output>

<gadget id="calculator">3_000 / 20</gadget>
<output>150</output>

<gadget id="calculator">150 - 1</gadget>
<output>149</output>

<gadget id="calculator">149 * 2</gadget>
<output>298</output>

<result>298</result>