Xiao Ming walks 3 kilometers per hour up the mountain, and 6 kilometers per hour down the original road. How many kilometers per hour does he walk up and down the mountain on average?
小明上山每小时走3千米，原路下山每小时走6千米．他上、下山平均每小时走多少千米．
<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 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/3) + (1/6)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">2 / (1/2)</gadget>
<output>4</output>

<result>4</result>