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A Mad tea party (Posted on 2002-04-30) Difficulty: 3 of 5
The Mad Hatter, March Hare and Dormouse are sitting down for a tea party. They sit at a table with twelve chairs, and twelve cups of tea.

Each day at six o'clock, everyone moves over two seats to the left or to the right (if any of those seats are free), then if there is tea in the cup at their seat, each one drinks it so the cup becomes empty.

After this, Alice comes and fills one of the empty cups on the table with tea again.

Prove that Alice can make sure that there are at least six full teacups on the table every day just before six.

See The Solution Submitted by levik    
Rating: 2.2000 (5 votes)

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my try Comment 2 of 2 |
since there are an even number of seats, each participant must stay on either odd or even number seats at all times (since they can only move two seats to the left or right).

Thus, with three persons, at least two must be on even or odd numbered seats. so, either even or odd numbers must be occupied by no more than one person.

alice needs only to check whether even or odd seats have the least occupants and fill cups only on those seats.
  Posted by ubergeek on 2002-12-25 22:15:13
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