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Die with the first rolled face removed

Question

You roll a fair six-sided die and then remove the face that came up, leaving five numbered faces. You keep rolling until one of those five remaining numbers appears. What is the expected total of the first roll and that second recorded value?

Short answer

77. If the first roll is ii, the second value is uniform on the other five faces, with mean 21−i5\tfrac{21-i}{5}. Averaging i+21−i5=21+4i5i+\tfrac{21-i}{5}=\tfrac{21+4i}{5} over i=1,…,6i=1,\dots,6 gives 77. Equivalently, the second value is marginally uniform on 11–66 by symmetry, so each roll averages 3.53.5.

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

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