I begin with a *.
Each turn I roll the die. If I rolled a , next turn I would roll a ; if I rolled a , next turn I would roll a ; if I rolled a , next turn I would roll a ; and so on.
If I roll a , I lose.
What is the probability that I lose before my fourth turn?
*Note that means 6-sided die, or is a 3-sided die (despite it not being possible from my knowledge).
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I feel certain that somewhere out there is a beautiful way to solve this that is currently eluding me. Until I think of it, here is the "brute force" solution:
The probability that you lose in exactly 1 turn is 6 1 .
The probability that you lose in exactly 2 turns is j = 2 ∑ 6 Prob( first roll is j ) × Prob( second roll is 1) = 6 1 j = 2 ∑ 6 j 1
The probability that you lose in exactly 3 turns is j = 2 ∑ 6 Prob( first roll is j ) × k = 2 ∑ j ( Prob( second roll is k ) × Prob( third roll is 1) ) = 6 1 j = 2 ∑ 6 k = 2 ∑ j j k 1
Putting this all together and using a script, the probability that you lose before the fourth turn is the sum of the above probabilities, which turns out to be 0 . 6 2 4 4 9