Install street lamps on both sides of a 300-meter-long bridge (both ends are installed), and install one lamp every 15 meters. Each street lamp costs 400 yuan. How much does it cost to buy these street lamps?
在一条300米长的大桥两旁装路灯(两端都装)，每隔15米装一盏，每盏路灯400元钱，买这些路灯共需要多少钱？
<gadget id="calculator">300 / 15</gadget>
<output>20</output>

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

<gadget id="calculator">21 * 2 * 400</gadget>
<output>16_800</output>

<result>16_800</result>