ship sails from port A to port B at 30 kilometers per hour. When returning, it travels against the current at 20 kilometers per hour. Find the average speed of the round trip.
一艘轮船从甲港开往乙港每小时30千米，返回时逆水，每小时20千米，求往返的平均速度．
<gadget id="calculator">8 / 150</gadget>
<output>4/75 = around 0.053333</output>

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

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