IMO 1986 LL CAN10

A set of n standard dice are shaken and randomly placed in a

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IMO 1986 LL CAN10

Origin: CAN

Problem

A set of n standard dice are shaken and randomly placed in a straight line. If n < 2r and r < s, then the probability that there will be a string of at least r, but not more than s, consecutive 1’s can be written as P/6s+2. Find an explicit expression for P.