ship sails from port A to port B with the current at a speed of 18 km/h; from port B back to port A travels against the current with a speed of 12 km/h. Find the average speed of the ship going back and forth.
一艘轮船从甲港驶往乙港是顺水行驶，航速是18千米/小时；从乙港返回甲港是逆水行驶，航速是12千米/小时，求这艘轮船往返的平均速度．
<gadget id="calculator">1 / 18</gadget>
<output>1/18 = around 0.055556</output>

<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/18) + (1/12)</gadget>
<output>5/36 = around 0.138889</output>

<gadget id="calculator">1 / (5/36)</gadget>
<output>36/5 = around 7.2</output>

<gadget id="calculator">2 * (36/5)</gadget>
<output>72/5 = around 14.4</output>

<result>72/5 = around 14.4</result>