The road between A and B has only uphill and downhill roads, and there is no flat road. A car goes uphill at 20 km/h and downhill at 30 km/h. It took the car 5 hours to get from A to B, and 7 hours to get from B to A. So, how long is this road in kilometers?
甲、乙两地之间的公路只有上坡和下坡，没有平路．一辆汽车上坡速度每小时20千米，下坡速度每小时30千米．这辆汽车从甲地到乙地用了5小时，从乙地到甲地用了7小时．那么，这条公路长多少千米．
<gadget id="calculator">5 + 7</gadget>
<output>12</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) + (1/30)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">12 / (1/12)</gadget>
<output>144</output>

<result>144</result>