A tourist climbs a mountain at a speed of 2km/h for going up and 6km/h for going down. Find his average speed for going up and down.
游客爬山，上山速度2km/h，下山速度6km/h，求他上、下山平均速度．
<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 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(1/2) + (1/6)</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">2 / (2/3)</gadget>
<output>3</output>

<result>3</result>