The students climb the mountain, and it takes 4 hours to go back and forth. It is known that the speed when going up the mountain is 3 kilometers per hour, and the speed when going down the mountain is 5 kilometers per hour. How many kilometers is the distance from the foot of the mountain to the top of the mountain?
同学们爬山，往返一次所用的时间是4小时，已知上山时的速度是3千米/时，下山时的速度是5千米/时，山脚到山顶的距离是多少千米？
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/3) + (1/5)</gadget>
<output>8/15 = around 0.533333</output>

<gadget id="calculator">2 / (8/15)</gadget>
<output>15/4 = around 3.75</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">(15/4) * 2</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>