A ship sails from port A to port B, traveling 30 kilometers per hour. When returning, it travels 20 kilometers per hour because of headwinds. What is the average speed of the ship in kilometers per hour?
一艘轮船从甲港驶往乙港，每小时行30千米，返回时因为逆水，每小时行20千米．这艘轮船往返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 / 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>