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Rolls needed given a six precedes any five

Question

A fair die is rolled repeatedly until the first 66 shows up. Conditional on no 55 having appeared before that first 66, what is the expected total number of rolls?

Short answer

33. Let TT be the first roll showing a 5 or a 6; TT is geometric with success probability 13\tfrac13, and whether that roll is a 5 or a 6 is independent of TT. Conditioning on it being a 6 makes the total number of rolls equal to TT, whose mean is still 33. (Simply deleting 5s from the die and answering 5 is a common mistake.)

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