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VN TST 2005 Question 2

VMO · VN TST 2005

Question 2

There are $n$ chairs on a circle, numbered $1$ through $n$. Select $k$ chairs. Two selected chairs are called adjacent when they are consecutive among the selected chairs. Find the number of selections for which between any two adjacent selected chairs there are at least three other chairs.

The supplied PDF does not state a time limit. This edition uses the user-approved 240 minutes per session, 480 minutes total for both sessions; this is an editorial convention. The supplied source has no question point values. Each problem uses 1 editorial point for import compatibility; this is not an official scoring scale. Essay grading is manual; the rubric is editorial. Import multipliers: correct 1, incorrect 0, blank 0.