Die-Morphosis

Probability Level pending

I begin with a D 6 D6 *.

Each turn I roll the die. If I rolled a 6 6 , next turn I would roll a D 6 D6 ; if I rolled a 5 5 , next turn I would roll a D 5 D5 ; if I rolled a 4 4 , next turn I would roll a D 4 D4 ; and so on.

If I roll a 1 1 , I lose.

What is the probability that I lose before my fourth turn?

*Note that D 6 D6 means 6-sided die, or D 3 D3 is a 3-sided die (despite it not being possible from my knowledge).


The answer is 0.6244907407.

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1 solution

Varsha Dani
Feb 22, 2019

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 1 6 \frac{1}{6} .

The probability that you lose in exactly 2 turns is j = 2 6 Prob( first roll is j ) × Prob( second roll is 1) = 1 6 j = 2 6 1 j \sum_{j=2}^{6} \mbox{Prob( first roll is } j \mbox{ ) } \times \mbox{Prob( second roll is 1)} = \frac{1}{6} \sum_{j=2}^{6} \frac{1}{j}

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) ) = 1 6 j = 2 6 k = 2 j 1 j k \sum_{j=2}^{6} \mbox{Prob( first roll is } j \mbox{ ) } \times \sum_{k=2}^{j} \big( \mbox{Prob( second roll is } k \mbox{ )} \times \mbox{Prob( third roll is 1)} \big) = \frac{1}{6} \sum_{j=2}^{6} \sum_{k=2}^{j} \frac{1}{jk}

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 0.62449 \boxed{0.62449}

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