A ship travels with a speed of 50 km/h along the current and returns with a speed of 30 km/h against the current. What is the average speed of its round trip in km/h?
一艘轮船去时顺水速度为50千米/时，返回时逆水速度为30千米/时，它往返的平均速度是多少千米/时．
<gadget id="calculator">1 / 50</gadget>
<output>1/50 = around 0.02</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/50) + (1/30)</gadget>
<output>4/75 = around 0.053333</output>

<gadget id="calculator">1 / (4/75)</gadget>
<output>75/4 = around 18.75</output>

<gadget id="calculator">2 * (75/4)</gadget>
<output>75/2 = around 37.5</output>

<result>75/2 = around 37.5</result>