Required Mathematical Intelligence

Chapter 153
A broken car has to travel two miles, one mile uphill and one mile downhill, with an average speed of 15 miles per hour on the uphill. Ask how fast it needs to go down the second mile to reach an average speed of 30 miles per hour. 45 miles per hour?Is it [-] miles?You have to think about it!

[答案：无论如何破车的平均速度也不可能达到30英里/小时。因为当平均速度为30英里/小时时，破车上、下山的总时间应为1/15小时。而破车上山就用了1/15小时。所以说破车的平均速度是达不到30英里/小时的。]
(End of this chapter)