There are two trains A and B. Train A is 93 meters long and travels 21 meters per second; train B is 126 meters long and travels 18 meters per second. Two trains are traveling in the same direction. At the beginning, the front of train A is level with the rear of train B. How long does it take before the rear of train A is level with the front of train B?
有甲、乙两列火车，甲火车长93米，每秒钟行驶21米；乙火车长126米，每秒钟行驶18米．两车同向而行，开始时甲火车的车头与乙火车的车尾相平．经过多长时间后，甲火车的车尾与乙火车的车头相平？
<gadget id="calculator">21 - 18</gadget>
<output>3</output>

<gadget id="calculator">126 / 3</gadget>
<output>42</output>

<result>42</result>