The car goes up the mountain at a speed of 36 km/h, travels for 5 hours to reach the top of the mountain, and immediately goes down the mountain on the same road at a speed of 45 km/h. How many hours does it take for the car to go up and down the mountain?
汽车上山的速度为36千米/小时，行5小时到达山顶，到山顶后立即按原路下山，速度为45千米/小时．汽车上山和下山共用多少小时？
<gadget id="calculator">5 / 45</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">36 * (1/9)</gadget>
<output>4</output>

<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<result>9</result>