Two trains are running towards each other on parallel rails. The lengths of the trains are 300 meters and 500 meters respectively, and the speeds are 90 kilometers per hour and 120 kilometers per hour. Find how long it takes for the two trains to meet and leave completely.
两列火车在平行的铁轨上相向而行，火车长分别是300米和500米，车速分别是每小时90千米和每小时120千米，求从两车相遇到完全离开需要多长时间．
<gadget id="calculator">300 + 500</gadget>
<output>800</output>

<gadget id="calculator">90 * 1_000</gadget>
<output>90_000</output>

<gadget id="calculator">90_000 / 60 / 60</gadget>
<output>25</output>

<gadget id="calculator">120 * 1_000</gadget>
<output>120_000</output>

<gadget id="calculator">120_000 / 60 / 60</gadget>
<output>100/3 = around 33.333333</output>

<gadget id="calculator">25 + (100/3)</gadget>
<output>175/3 = around 58.333333</output>

<gadget id="calculator">800 / (175/3)</gadget>
<output>96/7 = around 13.714286</output>

<result>96/7 = around 13.714286</result>