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标题: A funny question [打印本页]

作者: unusual_0 时间: 2008-10-26 08:29
标题: A funny question

i meet a funny question:

A monk go up the montain,the speed is 5km/h,the question is what is the speed when the monk go down in order to make the average speed equals 10km/h.

who can solve that?

[此贴子已经被作者于2008-10-27 1:21:35编辑过]

作者: finalsunny 时间: 2008-10-27 15:57

positive infinite speed?

P.S. what's the hurry? there's always enough time for a monk!

作者: xun 时间: 2008-11-3 01:48

5s=d(t)

s= time used to clime up

t= time used to go down

d= downward speed

And average speed= (5s+dt)/(s+t)=10s/(s+t)=10

s/(s+t)=1

t=0

There is no speed large enough to climb down to satisfy eaverage speed 10

作者: lianghao 时间: 2008-11-3 03:22
why 10s/(s+t)=10?

作者: kallyli 时间: 2008-11-18 07:40

Length while going up: L

Length while going down: r*L

speed while going down: x

everage speed V=(L+rL)/(L/5+rL/x)=[(r+1)*5x]/(x+5r)=10, x=10r/(r-1)

condition: x>0, so r-1>0, so r>1

r>0

=> r>1

conclusion: as long as r>1, x can be solved by replacing r by a number.

yes, this question is funny, as long as the monk chooses another longer route, he can reach the average speed of 10. Interesting.

[此贴子已经被作者于2008-11-18 7:59:08编辑过]

作者: Bonhomie 时间: 2008-11-19 09:43
恩 funny 开始默认上山路程等于下山路程了，呵呵

作者: tinyTINY 时间: 2008-11-19 17:33

如果路程相同, 这个题目也太无聊了吧, 估计世界上99%的人都会得出这个答案, 楼主还问个什么劲啊.

楼主去哪了??

[此贴子已经被作者于2008-11-19 17:41:36编辑过]

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