A car drives in a mountainous area. It takes 5 hours to go up the mountain at an average of 30 kilometers per hour. When going down the mountain, it takes only 3 hours to go back the same way. What is the average speed of the car in miles per hour?
一辆汽车在山区行驶，上山平均每小时行30千米，用了5小时；下山时按原路返回，只用了3小时．这辆汽车往返的平均速度是多少千米/小时．
<gadget id="calculator">5 + 3</gadget>
<output>8</output>

<gadget id="calculator">2 / 8</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">30 * 5 * (1/4)</gadget>
<output>75/2 = around 37.5</output>

<result>75/2 = around 37.5</result>