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 the easiest way 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?