There are two trains, one is 102 meters long and travels 20 meters per second; the other is 120 meters long and travels 17 meters per second. Two cars are going in the same direction, how many seconds does it take from the first train catching up with the second train to the two cars leaving?
有两列火车，一列长102米，每秒行20米；一列长120米，每秒行17米．两车同向而行，从第一列车追及第二列车到两车离开需要多少秒？
<gadget id="calculator">102 + 120</gadget>
<output>222</output>

<gadget id="calculator">20 - 17</gadget>
<output>3</output>

<gadget id="calculator">222 / 3</gadget>
<output>74</output>

<result>74</result>