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Joined: Jun 11, 2015
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June 11th, 2015 at 6:42:41 AM permalink
Is there a formula to determine the probability of a result, based on REROLL?

For example: I roll a 20 sided die, and I need a result of 19-20. However, I have a power that lets me RE-ROLL one time and ignore the previous result. What would the probability be? What if the power lets me RE-ROLL multiple times?

What about a 1d6? or a 2d6? Is there a mathematical fomula for this?

NOTE: This is not the same as roll multiple-times simultaneously, which has different odds.
Joined: Mar 11, 2010
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June 11th, 2015 at 7:55:08 AM permalink
Sum: odds of first roll success + (odds of first roll failure * odds of second roll success) + (odds of second roll failure * odds of third roll success)...(odds of n-1 roll failure * odds of n roll success)
"So as the clock ticked and the day passed, opportunity met preparation, and luck happened." - Maurice Clarett
Joined: Jun 22, 2011
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June 11th, 2015 at 10:08:24 AM permalink
If you are allowed up to N total rolls (i.e. your first roll plus N-1 rerolls), then the probability of failure = (1 - p)N, where p is the probability of success (i.e. to roll a 19 or 20 on a d20, p = 18/20 = 0.9), and the probability of success = 1 minus that, or (1 - p)N.

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