There are 10 students who went to climb the mountain in the morning, and they finished the 15-kilometer activity in 5 hours. It is known that their speed when they go up the mountain is 2.4 kilometers per hour, then what is the average speed of them when they return down the mountain by the original road in kilometers per hour.
有10名同学上午去爬山，他们用5个小时结束了全程15千米的活动．己知他们上山时的速度为每小时2.4千米，则他们按原路下山返回时的平均速度是每小时多少千米．
<gadget id="calculator">15 / 2 / 2.4</gadget>
<output>3.125</output>

<gadget id="calculator">5 - 3.125</gadget>
<output>1.875</output>

<gadget id="calculator">7.5 / 1.875</gadget>
<output>4</output>

<result>4</result>