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 1 meter per second, and the speed of the cyclist is 3 meters per second. At this time, a train came from behind them. It took 20 seconds for the train to pass by pedestrians and 30 seconds for the train to pass by cyclists. How long is the body of this train?
铁路旁一条平行小路上，有一行人与一骑车人同时向南行进，行人速度为每秒1米，骑车人速度为每秒3米．这时有一列车从他们背后开过来，火车通过行人用20秒，通过骑车人30秒．这列火车的车身长多少米？
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) - (1/30)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">2 / (1/60)</gadget>
<output>120</output>

<result>120</result>