A ship sails from A to B at 30 kilometers per hour, and then returns on the same route at 60 kilometers per hour. What is the average speed of the ship in kilometers per hour?
一艘轮船从甲地到乙地每小时航行30千米，然后按原路返回，每小时航行60千米，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 / 60</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/30) + (1/60)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">2 / (1/20)</gadget>
<output>40</output>

<result>40</result>