Wang Fei went to the library on the mountain to borrow books. He went up the mountain and traveled 3 kilometers per hour, and returned from the original road at 6 kilometers per hour. Find his average speed up and down the hill.
王飞到山上图书馆借书，他上山每小时行3千米，从原路返回，每小时行6千米．求他上下山的平均速度．
<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 / 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>