There are two trains, the express train is 255 meters long and travels 42 meters per second, the slow train is 195 meters long and travels 27 meters per second, as if traveling in the same direction, how many seconds does it take for the express train to catch up with the slow train and overtake it?
有两列火车，快车长255米，每秒行42米，慢车长195米，每秒行27米，如同向而行，快车追上慢车并超错而过，需多少秒．
<gadget id="calculator">255 + 195</gadget>
<output>450</output>

<gadget id="calculator">42 - 27</gadget>
<output>15</output>

<gadget id="calculator">450 / 15</gadget>
<output>30</output>

<result>30</result>