If a person goes up the mountain at a speed of 2 kilometers per hour and goes down the mountain at a speed of 4 kilometers per hour, what is the average speed of his going up and down the mountain in kilometers per hour?
某人上山的速度为每小时2千米，下山的速度为每小时4千米，那么他上山、下山的平均速度是每小时多少千米．
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(1/2) + (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">2 / (3/4)</gadget>
<output>8/3 = around 2.666667</output>

<result>8/3 = around 2.666667</result>