After Xiaoming climbed to the top of the mountain, he went down the mountain along the original road. It took 4 hours to go up and down the mountain. It is known that the speed of going down the mountain is 7.5 kilometers per hour after going up the mountain in 2.4 hours. Find the speed of going up the mountain.
小明登上山顶之后沿原路下山，上下山共用4小时，已知上山用了2.4小时下山的速度是每小时7.5千米，求上山的速度．
<gadget id="calculator">4 - 2.4</gadget>
<output>1.6</output>

<gadget id="calculator">7.5 / 2.4</gadget>
<output>3.125</output>

<gadget id="calculator">1.6 * 3.125</gadget>
<output>5</output>

<result>5</result>