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The Monk

A monk talks during supper. As penance, he has to climb a mountain.
He leaves at 9 o'clock in the morning and reaches the top of the mountain at midday. He rests for a night under the stars and sets out again the next morning at 9 o'clock. Taking the same path down, he gets to the bottom at 11 o'clock.
Is there any place on the path that he passes through at the same time on both days?
How can the existence or nonexistence of such a place be demonstrated?

I will post the answer below later.

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