Xiaoli climbs a mountain. The whole distance from the bottom to the top of the mountain is 8.4 kilometers. Her speed of going up the mountain is 2 kilometers per hour, and her speed of going down the mountain is 3 kilometers per hour. Find Xiaoli's average speed of going up and down the mountain.
小丽爬一座山，从山底到山顶全程有8.4千米，她上山的速度是每小时2千米，下山的速度是每小时3千米，求小丽上下山的平均速度．
<gadget id="calculator">8.4 / 2</gadget>
<output>4.2</output>

<gadget id="calculator">8.4 / 3</gadget>
<output>2.8</output>

<gadget id="calculator">4.2 + 2.8</gadget>
<output>7</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<gadget id="calculator">8.4 * (2/7)</gadget>
<output>2.4</output>

<result>2.4</result>