A person is walking along the road around the city at a speed of 3 kilometers per hour. Every 7 minutes, a car catches up with him from behind him, and every 5 minutes, he encounters an oncoming car. The time between cars is the same, and the speed is also the same. How many kilometers does the car travel per hour?
某人以每小时3千米的速度沿着环城汽车道旁前进．每7分钟有一辆汽车从他后面追上他，每5分钟又与迎面开来的汽车相遇一次．汽车间隔时间相同，速度也相同．汽车每小时行多少千米？
<gadget id="calculator">7 - 5</gadget>
<output>2</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

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

<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<gadget id="calculator">21 - 3</gadget>
<output>18</output>

<result>18</result>