A car travels 20 kilometers per hour up the mountain, takes 3 hours to reach the summit, and then goes down the mountain and returns by the same route. It takes 2 hours. Find the average speed of the car going back and forth.
汽车上山每小时行20千米，3小时登顶，下山按原路返回，用了2小时，求汽车往返的平均速度．
<gadget id="calculator">3 + 2</gadget>
<output>5</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">20 * 3 * (2/5)</gadget>
<output>24</output>

<result>24</result>