Xiaoqiang and his parents went hiking on Sunday, and it took 2.4 hours to go up the mountain. After going down the mountain along the original road, they found that it took 4 hours to go up and down the mountain. It is known that their downhill speed is 7.5 kilometers per hour, what is their uphill speed? (solution with equation)
星期天小强和爸爸妈妈去登山，上山用了2.4小时，沿原路下山后，他们发现上下山一共用了4小时．已知他们下山的速度是每小时7.5千米，上山的速度是多少？(用方程解答)
<gadget id="calculator">7.5 * 4</gadget>
<output>30</output>

<gadget id="calculator">7.5 * 2.4</gadget>
<output>18</output>

<gadget id="calculator">30 - 18</gadget>
<output>12</output>

<gadget id="calculator">12 / 2.4</gadget>
<output>5</output>

<result>5</result>