The distance from the foot of the mountain to the top of the mountain is 4 kilometers. A person goes up the mountain at a speed of 1 kilometer per hour. After he climbs to the top of the mountain, he immediately goes down the same road. It is known that the average speed of going up and down the mountain is 1.6 kilometers, how many kilometers per hour does this person go down the mountain?
山脚到山顶有4千米，一个人以每小时1千米的速度上山，他爬到山顶后立即原路下山．已知上山和下山的平均速度是1.6千米，这个人下山每小时走多少千米？
<gadget id="calculator">2 / 1.6</gadget>
<output>1.25</output>

<gadget id="calculator">4 * 1.25</gadget>
<output>5</output>

<gadget id="calculator">4 / 1</gadget>
<output>4</output>

<gadget id="calculator">5 - 4</gadget>
<output>1</output>

<result>4</result>