Xiao Ming climbs Nanshan Mountain, going up the mountain at 3 kilometers per hour, and going down the mountain at 5 kilometers per hour. Find his average speed for climbing the mountain.
小明爬南山，上山每小时走3千米，下山每小时走5千米．求他爬山的平均速度．
<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>