person is moving along the trolleybus lane around the city at a speed of 3 kilometers per hour. Every 7 minutes, a tram catches up with him from behind, and every 5 minutes, he meets the oncoming tram again. It is known that the distance between the trams on the track is equal and the speed is the same, so the speed of the tram is how many kilometers per hour.
某人以每小时3千米的速度沿着环城无轨电车道旁前进，每7分钟有一辆电车从他后面追上他，每5分钟又与迎面开来的电车相遇一次．已知轨道上的电车间隔相等，速度相同，则电车速度为每小时行多少千米．
<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">(5/7) * 3</gadget>
<output>15/7 = around 2.142857</output>

<gadget id="calculator">3 + (15/7)</gadget>
<output>36/7 = around 5.142857</output>

<gadget id="calculator">1 - (5/7)</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">(36/7) / (2/7)</gadget>
<output>18</output>

<result>18</result>