When a car goes up the mountain, it travels 3 kilometers in 2 hours. It travels at this speed for 7 hours to reach the mountain, and when it returns down the mountain, it travels 3.5 kilometers per hour. So how many hours can return to the original place?
一辆汽车上山时，2小时行了3千米，用这样的速度行了7小时到达山上，下山返回时每小时行3.5千米．这样几小时可返回原地？
<gadget id="calculator">3 / 2</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">(3/2) * 7</gadget>
<output>21/2 = around 10.5</output>

<gadget id="calculator">(21/2) / 3.5</gadget>
<output>3</output>

<result>3</result>