A person climbs a hill, travels 6 kilometers per hour when going up, and travels 9 kilometers per hour when descending. It takes 5 hours to go back and forth. Ask for the length of the mountain road.
某人爬山坡，上去每小时行6千米，下来时每小时行9千米，往返用了5小时，求山路长．
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">(1/6) + (1/9)</gadget>
<output>5/18 = around 0.277778</output>

<gadget id="calculator">5 / (5/18)</gadget>
<output>18</output>

<result>18</result>