The express train is 102 meters long and travels 21 meters per second; the slow train is 117 meters long and travels 12 meters per second. The two cars are running in the same direction on the track; from the time when the difference between the front of the express train and the rear of the slow train is 105 meters, the timing starts, and when the express train catches up with the slow train and leaves the front of the slow train, how many minutes have elapsed?
快车长102米，每秒行21米；慢车长117米，每秒行12米．两车在轨道上同向行驶；从快车车头与慢车车尾差105米，开始计时，到快车追上慢车离开慢车车头，问共经过多少分钟？
<gadget id="calculator">105 + 102 + 117</gadget>
<output>324</output>

<gadget id="calculator">21 - 12</gadget>
<output>9</output>

<gadget id="calculator">324 / 9</gadget>
<output>36</output>

<gadget id="calculator">36 / 60</gadget>
<output>3/5 = around 0.6</output>

<result>3/5 = around 0.6</result>