A car travels between A and B with an upward speed of 20 km/h and a downward speed of 30 km/h, find the average speed of the round trip.
汽车往返于甲、乙两地之间，上行速度为每小时20千米，下行速度为每小时30千米，求往返的平均速度．
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

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

<gadget id="calculator">(1/20) + (1/30)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">2 / (1/12)</gadget>
<output>24</output>

<result>24</result>