(adsbygoogle = window.adsbygoogle || []).push({}); A d20 probability question....

If you know what a d20 is, perhaps you can help out a fellow gamer who is trying to make the gaming world a better place. I'm trying to create a tool that helps people design balanced skill challenges for the new 4E D&D game.

To create this tool for fellow gamers, I need to figure out theeasiestway to calculate the chance of succeeding on a probability roll X times before failing Y times. Preferably, I'd like this to be something I could calculate in excel.

I'd like to be able to figure out things like:

* The chance of rolling 5 numbers greater than or equal to 8 before I roll 3 numbers less than 8 on a twenty sided die (a d20).

* The chance of rolling 2 numbers greater than or equal to 14 before I roll 4 numbers less than 14 on a twenty sided die.

* The chance of rolling 4 numbers greater than or equal to 12 before I roll 3 numbers less than 12 on a twenty sided die.

I don't need to know how to calculate this for more than 10 rolls of the dice (# of successes needed + number of failures to avoid = 11).

Can anyone help me out and explain it in a way that would make sense to someone who's most recent experience in this arena is a basic probaility class 15 years ago?

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# A d20 probability question

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