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Four dogs in a row at a round table

Question

Six dogs and six cats are placed at random in the 1212 seats around a round table. What is the probability that some four consecutive seats are all occupied by dogs?

Short answer

311\tfrac{3}{11}. A run of four or more dogs means that, going clockwise from some seat, we see a cat followed by four dogs. With only six dogs, at most one seat can start such a pattern, so these 12 events are disjoint. Each has probability 612⋅611⋅510⋅49⋅38=144\tfrac{6}{12}\cdot\tfrac{6}{11}\cdot\tfrac{5}{10}\cdot\tfrac{4}{9}\cdot\tfrac{3}{8}=\tfrac{1}{44}, and the total is 12⋅144=31112\cdot\tfrac1{44}=\tfrac3{11}.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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