A ship goes back and forth between A and B. It travels 30 kilometers per hour when going there, and travels 20 kilometers per hour when it returns. What is the average speed of one round trip in kilometers per hour?
轮船往返甲、乙两地之间，去时每小时行30千米，返回时每小时行20千米，往返一次的平均速度是每小时多少千米
<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

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

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

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

<result>24</result>