A car drives across an arch bridge. The uphill and downhill distances of the bridge are equal. The car goes uphill at 60 kilometers per hour and downhill at 80 kilometers per hour. What is the average speed of the car across the bridge in kilometers?
一辆汽车驶过一座拱桥，桥的上坡与下坡的路程相等，汽车上坡每小时60千米，下坡每小时80千米，汽车过桥的平均速度是多少千米？
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">1 / 80</gadget>
<output>1/80 = around 0.0125</output>

<gadget id="calculator">(1/60) + (1/80)</gadget>
<output>7/240 = around 0.029167</output>

<gadget id="calculator">2 / (7/240)</gadget>
<output>480/7 = around 68.571429</output>

<result>480/7 = around 68.571429</result>