A car goes up a hill at a speed of 30 km/h, and travels back down the hill at a speed of 50 km/h, find the average speed of the car going back and forth.
一辆汽车上山时速度是每小时30千米，沿原路返回下山速度是每小时50千米，求汽车往返的平均速度．
<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>