On a parallel path next to a railway, a group of pedestrians and a cyclist are traveling south at the same time. The speed of the pedestrians is 3.6 kilometers per hour, and the speed of the cyclists is 10.8 kilometers per hour. At this time, there is a train coming from behind them. It takes 15 seconds for the train to pass by pedestrians and 20 seconds for the train to pass by cyclists. How long is this train?
铁路旁的一条平行小路上，有一行人与骑车人同时向南行进，行人速度为每小时3.6千米，骑车人速度为每小时10.8千米．这时，有一列火车从他们背后开过来，火车通过行人用15秒，通过骑车人用20秒．这列火车的车长多少米？
<gadget id="calculator">3 * 20</gadget>
<output>60</output>

<gadget id="calculator">60 - 15</gadget>
<output>45</output>

<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<gadget id="calculator">45 / 5</gadget>
<output>9</output>

<gadget id="calculator">9 - 3</gadget>
<output>6</output>

<gadget id="calculator">6 * 20</gadget>
<output>120</output>

<result>120</result>