A person travels from city A to city B at a speed of 9 km/h, and from city B back to city A at a speed of 7 km/h, what is the average speed of the person going back and forth in km/h
某人由A市往B市的速度为9千米/小时，由B市返回A市的速度为7千米/小时，问此人往返的平均速度是多少千米/小时
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">(1/9) + (1/7)</gadget>
<output>16/63 = around 0.253968</output>

<gadget id="calculator">2 / (16/63)</gadget>
<output>63/8 = around 7.875</output>

<result>63/8 = around 7.875</result>