Li Qiang climbed a mountain. He traveled 4 kilometers per hour when he went up the mountain, and 5 kilometers per hour when he returned to the mountain along the original road. It took him 4.5 hours to go up and down the mountain. How many kilometers is the distance from the bottom of the mountain to the top of the mountain?
李强爬一座山，上山时每小时行4千米，沿原路返回下山时每小时行5千米，上山、下山共用了4.5小时，山下距山顶多少千米？
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) + (1/5)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">4.5 / (9/20)</gadget>
<output>10</output>

<result>10</result>