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Rolls until consecutive values differ by two

Question

A fair six-sided die is rolled repeatedly. What is the expected number of rolls until two successive rolls differ by exactly 22?

Short answer

17/317/3. Group faces by how many follow-ups finish the game: faces 1,2,5,61,2,5,6 have one partner and faces 3,43,4 have two. Writing first-step equations for these two states, E1=1+23E1+16E2E_1=1+\tfrac23E_1+\tfrac16E_2 and E2=1+13E1+13E2E_2=1+\tfrac13E_1+\tfrac13E_2, gives E1=5E_1=5, E2=4E_2=4, and the total is 1+23E1+13E2=1731+\tfrac23E_1+\tfrac13E_2=\tfrac{17}{3}.

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

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