A and B start climbing from the foot of the mountain at the same time, and descend immediately after reaching the top of the mountain. Both of them descended at twice the speed of their ascent. When A reaches the top of the mountain, B is still 400 meters away from the top of the mountain. When A returns to the foot of the mountain, B is just halfway down the mountain. Find the distance from the top of the mountain to the foot of the mountain.
甲、乙两人同时从山脚开始爬山，到达山顶后就立即下山．他们两人下山的速度都是各自上山速度的2倍．甲到山顶时，乙距山顶还有400米，甲回到山脚时，乙刚好下到半山腰．求从山顶到山脚的距离．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 + (1/2)</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">(1/2) / 2</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 + (1/4)</gadget>
<output>5/4 = around 1.25</output>

<gadget id="calculator">(3/2) / (5/4)</gadget>
<output>6/5 = around 1.2</output>

<gadget id="calculator">1 / (6/5)</gadget>
<output>5/6 = around 0.833333</output>

<gadget id="calculator">1 - (5/6)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">400 / (1/6)</gadget>
<output>2_400</output>

<result>2_400</result>