There is a mountain road, a car travels 30 kilometers per hour when going up the mountain, and travels 50 kilometers per hour when returning from the original road down the mountain, then the average speed of this car going up and down the mountain is how many kilometers per hour.
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，那么这辆汽车上下山的平均速度是每小时多少千米．
<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>