Cats and dogs on the street

MediumBrainteasers~6m

Walking down a very long street, you observe that 67\tfrac67 of the cats are immediately followed by a dog, while 14\tfrac14 of the dogs are immediately followed by a cat. In the long run, what proportion of the animals are dogs?

Show hints (2)+
  1. Count adjacencies: cat\todog transitions number 67C\tfrac67 C; dog\tocat transitions number 14D\tfrac14 D.
  2. In a long line these transition counts must match. Set them equal and solve for D/(C+D)D/(C+D).

Answer

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0.7742 (± 0.001)

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Asked at: Jane Street, Optiver

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