A person travels an average of 35 kilometers per hour when going up the mountain from the foot of the mountain, and an average of 40 kilometers per hour when descending from the top of the mountain along the same road. It takes 7.5 hours to go back and forth.
某人从山脚上山平均每小时行35千米，从山顶沿原路下山时平均每小时行40千米，往返一次共用7.5小时，山脚到山顶的距离是多少千米？
<gadget id="calculator">1 / 35</gadget>
<output>1/35 = around 0.028571</output>

<gadget id="calculator">1 / 40</gadget>
<output>1/40 = around 0.025</output>

<gadget id="calculator">(1/35) + (1/40)</gadget>
<output>3/56 = around 0.053571</output>

<gadget id="calculator">7.5 / (3/56)</gadget>
<output>140</output>

<result>140</result>