There is a mountain road. A car travels 30 kilometers per hour when going up the mountain, and 50 kilometers per hour when going back down the mountain from the original road. What is the average speed of the car going up and down the mountain?
有一条山路，一辆汽车上山时每小时行30千米，从原路返回下山时每小时行50千米，求汽车上山、下山的平均速度是多少？
<gadget id="calculator">1 * 2</gadget>
<output>2</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">1 / 50</gadget>
<output>1/50 = around 0.02</output>

<gadget id="calculator">(1/30) + (1/50)</gadget>
<output>4/75 = around 0.053333</output>

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

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