Enigman said:
oops...sorry.
Okay rephrasing the question:
A monk climbs to the top of a certain mountain starting at sun-rise with unequal speeds and random stops of random durations, he reaches the top at the sunset of the same day. After meditating there for a week, he starts climbing down the mountain at the sun-rise with unequal speeds and random stops. Assuming that he follows the exact same path for both journeys prove that there exists a time of day where the monk was at the same position on the path for both journeys.
(Sorry again...should never trust the net to give accurate statements...I should have phrased it myself but I just couldn't remember the wordings...)
EDIT:Although the way the enigma was previously stated is solvable and is similar to what I had in mind...I can't decide which phrasing is easier...but 13 day thing may make things more complicated so let's stick to this one
Actually, in order for it to be
guaranteed that the monk will be at the same place, the same time of day, I'm pretty sure that at least one of the trips (either ascending or descending) must be greater than or equal to one day.
For example, it wouldn't hold if he makes it to the top of the mountain by sunset
the same day he started, then started down at sunrise by first taking a long break (at the top of the mountain) until after sunset then made it all the way down the mountain before dawn.
Enigman said:
The only essential thing is that both of the journeys start at the same time of the day and the monk does complete both trips; anything else is extraneous.
There is no math involved, just some elegant reasoning.
I won't use any mathematical rigor. Instead I will prove it graphically.
Here is essentially a proof (albeit a very non-rigorous one). In order for this proof to always work, no matter what, at least one of the trips must be greater than or equal to a day.
We plot the distance to the mountain top during the monk's ascent.
I made it so he took about three days to reach the top. But all that is necessary is that it takes a day or more. Actually, it can be less than a day as long as the decent takes over a day, but since the original enigma specified that he descends faster, his ascent must take more than a day.
We can also plot his descent referencing the time at which he started his decent. We can overlay the plots on one another. In other words, we just line up 6:30 AM with one day when he was ascending with 6:30 AM on a day he was descending.
And since the span of the y-axis is the same for both curves, it is guaranteed that there be at least one time of day where the monk is in the same place.
[Edit: 'looks like Zooby beat me to the answer.]