cyclist is doing pre-race practice between two cities A and C 10km apart, and wants to ride at an average speed of 40km per hour. Village B is exactly located at the midpoint of the distance between A and C. After a long climb from A, you will reach B. When the cyclist reaches B, he calculates that the average speed so far is only 25 km/h. If he wants to reach an average speed of 40 km/h, how many kilometers per hour does he ride on the downhill road from B to C?
一位自行车手在相距10km的两个城市A与C之间作赛前练习，想要骑出每小时40km的平均速度．B村恰好位于A与C路程的中点，由A做一段长途爬坡之后会到达B．当自行车手到达B时，他计算到目前为止，平均速度只有每小时25km．如果他要达到每小时40km的全程平均速度，则他在由B往C的下坡路上平均每小时骑多少千米？
<gadget id="calculator">10 / 2</gadget>
<output>5</output>

<gadget id="calculator">10 / 40</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">5 / 25</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) - (1/5)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">5 / (1/20)</gadget>
<output>100</output>

<result>100</result>