Xiao Ming travels 3 kilometers per hour when he goes up the mountain, and travels 5 kilometers per hour when he returns to the original road. What is the average speed of Xiao Ming's round trip in kilometers per hour?
小明上山时每小时行3千米，原路返回时每小时行5千米，小明往返的平均速度是多少千米/时．
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/3) + (1/5)</gadget>
<output>8/15 = around 0.533333</output>

<gadget id="calculator">2 / (8/15)</gadget>
<output>15/4 = around 3.75</output>

<result>15/4 = around 3.75</result>