The road from the top of the mountain to the bottom of the mountain is 72 kilometers long. It takes 4 hours for a car to go up the mountain to reach the top of the mountain, and it takes only 2 hours to reach the foot of the mountain when going down the mountain. Find the average speed of the car going back and forth.
从山顶到山底的路长72千米，一辆汽车上山，需要4小时到达山顶，下山沿原路返回，只用2小时到达山脚，求这辆汽车往返的平均速度．
<gadget id="calculator">4 + 2</gadget>
<output>6</output>

<gadget id="calculator">2 / 6</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">72 * (1/3)</gadget>
<output>24</output>

<result>24</result>