Two trains travel towards each other on two sets of parallel tracks. Car A is 720 meters long and its speed is 28 m/s; car B is 900 meters long and its speed is 26 m/s. How long does it take for the two cars to meet at the front and leave at the rear?
两列火车在两组互相平行的轨道上相向行驶．甲车长720米，速度是28米/秒；乙车长900米，速度是26米/秒．从两车车头相遇到车尾离开，共需要多少时间？
<gadget id="calculator">720 + 900</gadget>
<output>1_620</output>

<gadget id="calculator">28 + 26</gadget>
<output>54</output>

<gadget id="calculator">1_620 / 54</gadget>
<output>30</output>

<result>30</result>