From A to B, travel 20 kilometers per hour, return from the original road, travel 30 kilometers per hour, what is the average speed of going back and forth from A to B in kilometers per hour?
从甲到乙，每小时行20千米，从原路返回，每小时行30千米，往返甲乙的平均速度是每小时多少千米．
<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>