Install street lamps (both ends) on both sides of a road, one lamp every 30 meters, a total of 42 lamps are installed, how long is this road?
在一条马路的两边安装路灯(两端都装)，每隔30米装1盏，一共装了42盏，这条马路有多长？
<gadget id="calculator">42 / 2</gadget>
<output>21</output>

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

<gadget id="calculator">20 * 30</gadget>
<output>600</output>

<result>600</result>