Three passengers take the same train, but the front and rear carriages are different. When the train began to enter the station, they each counted the street lights on the platform, and they counted 15, 12, and 7 respectively. When the train started again, they continued to count street lights, and when the train pulled out of the platform, one of the passengers counted three times as many street lights as one. How many street lights are there in the station?
三位旅客乘坐同一列火车，只是前后车厢不同．当火车开始进站的时候，他们各自数月台上的路灯，结果分别数得15盏、12盏、7盏．当列车再次启动时，他们开始继续数路灯，到列车驶出月台时，他们中的一位旅客数得得路灯数是一个的3倍．站内有多少盏路灯？
<gadget id="calculator">3 * 15</gadget>
<output>45</output>

<gadget id="calculator">45 - 7</gadget>
<output>38</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">38 / 2</gadget>
<output>19</output>

<result>19</result>