Xiaogang goes up the mountain at 2.4 kilometers per hour and arrives in 4.9 hours, and goes down the mountain along the original road at 2.8 kilometers per hour. How long does it take to go down the mountain?
小刚上山每小时行2.4千米，4.9小时到达，沿着原路下山每小时行2.8千米，下山需要多长时间？
<gadget id="calculator">4.9 / 2.8</gadget>
<output>1.75</output>

<gadget id="calculator">2.4 * 1.75</gadget>
<output>4.2</output>

<result>4.2</result>