A car travels from A to B with a favorable wind and takes 4 hours. When it returns against the wind, it travels 25 kilometers per hour and takes 6 hours. What is the average speed of the car going back and forth?
汽车从甲地到乙地，去时顺风，用了4小时，返回时逆风，每小时行25千米，用了6小时，求汽车往返的平均速度？
<gadget id="calculator">4 + 6</gadget>
<output>10</output>

<gadget id="calculator">2 / 10</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">25 * 6 * (1/5)</gadget>
<output>30</output>

<result>30</result>