An express train and a slow train are running towards each other on two parallel tracks. The length of the express train is 420 meters and the length of the slow train is 525 meters. The time for people sitting on the express train to see the slow train passing by is 15 seconds. How many seconds did it take to see the express train passing by?
一列快车与一列慢车在两条平行轨道上相向而行，快车车长420米，慢车车长525米，坐在快车上的人看见慢车驶过的时间是15秒，那么坐在慢车上的人看见快车驶过的时间是多少秒．
<gadget id="calculator">525 / 15</gadget>
<output>35</output>

<gadget id="calculator">420 / 35</gadget>
<output>12</output>

<result>12</result>