Mingming is sitting on the train. When the train crosses the bridge, it takes 30 seconds from when he sees the head of the bridge to when he sees the end of the bridge. The bridge is 600 meters long. Then, he saw a truck coming towards him. It took him 20 seconds from the front of the truck to the rear. The length of the truck is 700 meters. What is the speed of this truck in meters per second?
明明坐在列车上，火车过桥时，从他看到桥头起到看到桥尾时间是30秒，这座桥长600米．而后，他又看到迎面开来一列货车，他从看到货车车头至车尾共用了20秒，货车长700米，这列货车速度是每秒钟多少米．
<gadget id="calculator">700 / 20</gadget>
<output>35</output>

<gadget id="calculator">600 / 30</gadget>
<output>20</output>

<gadget id="calculator">35 - 20</gadget>
<output>15</output>

<result>15</result>