The average speed of a car from city A to city B is 40 km/h, and the average speed from city B to city A is 24 km/h. What is the average speed of this car going back and forth between A and B in kilometers per hour?
一辆汽车从甲城到乙城的平均速度是40千米/小时，从乙城到甲城的平均速度是24千米/小时．则这辆汽车往返甲、乙两城的平均速度是多少千米/小时
<gadget id="calculator">1 / 40</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">1 / 24</gadget>
<output>1/24 = around 0.041667</output>

<gadget id="calculator">(1/40) + (1/24)</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">2 / (1/15)</gadget>
<output>30</output>

<result>30</result>