A ship travels 30 kilometers per hour from A to B, and then returns on the same route. If the average speed of the round trip is 40 kilometers, how many kilometers should it travel per hour when returning.
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，若想往返的平均速度为40千米，则返回时每小时应航行多少千米．
<gadget id="calculator">1 * 2</gadget>
<output>2</output>

<gadget id="calculator">2 / 40</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>1/60 = around 0.016667</output>

<gadget id="calculator">1 / (1/60)</gadget>
<output>60</output>

<result>60</result>