The car climbed over a big mountain along the Panshan Highway, with a total length of 120 kilometers. It is known that the speed of the car going up the hill is 30 kilometers per hour, and the speed of going down the hill is 40 kilometers per hour. How many hours does it take for a car to go back and forth.
汽车沿着盘山公路翻越一座大山，全长120千米．已知汽车上山的速度是每小时30千米，下山的速度是每小时40千米．汽车往返一次需多少小时．
<gadget id="calculator">120 / 30</gadget>
<output>4</output>

<gadget id="calculator">120 / 40</gadget>
<output>3</output>

<gadget id="calculator">4 + 3</gadget>
<output>7</output>

<result>7</result>