A funny question

unusual_0

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]

finalsunny

positive infinite speed?

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

xun

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

lianghao

why 10s/(s+t)=10?

kallyli

.

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]

Bonhomie

恩 funny 开始默认上山路程等于下山路程了，呵呵

tinyTINY

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]