A ship travels from A to B, traveling 75 kilometers per hour when going, and traveling 50 kilometers per hour when returning. Find the average speed of the boat to and fro in kilometers per hour.
一艘船从甲地到乙地，去时每小时行75千米，回来时每小时行50千米，求这条船往返的平均速度是每小时多少千米．
<gadget id="calculator">1 * 2</gadget>
<output>2</output>

<gadget id="calculator">1 / 75</gadget>
<output>1/75 = around 0.013333</output>

<gadget id="calculator">1 / 50</gadget>
<output>1/50 = around 0.02</output>

<gadget id="calculator">(1/75) + (1/50)</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">2 / (1/30)</gadget>
<output>60</output>

<result>60</result>