Xiaofeng travels 3 kilometers per hour uphill and 5 kilometers per hour downhill. One day, he walked from the foot of the mountain to the top of the mountain, and then immediately returned to the foot of the mountain, spending a total of 3.6 hours. Find the length of the mountain road in kilometers.
小风上山每小时行3千米，下山每小时行5千米．某日他从山脚走到山顶，再立即返回山脚，一共用了3.6小时．求山路长多少千米．
<gadget id="calculator">5 + 3</gadget>
<output>8</output>

<gadget id="calculator">5 / 8</gadget>
<output>5/8 = around 0.625</output>

<gadget id="calculator">3.6 * (5/8)</gadget>
<output>2.25</output>

<gadget id="calculator">3 * 2.25</gadget>
<output>6.75</output>

<result>6.75</result>