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Feb 5 at 21:21 comment added Eddymage @GentlePurpleRain Thanks for this, I added to the answer. Indeed, the difference of just 1 between the terms in the ratio ringed a bell, but I did not pay enough attention.
Feb 5 at 21:20 history edited Eddymage CC BY-SA 4.0
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Feb 5 at 20:38 comment added GentlePurpleRain The number of faces on the die is actually irrelevant. Only the number of possible "success" results is important. (If you're trying to get 90 or more on a d100 vs. 10 or more on a d20, the probabilities are exactly the same.)
Feb 5 at 20:37 comment added GentlePurpleRain You can simplify that final formula even more. Without loss of generalization, you can say that instead of wanting to get a number that is "at least \$t\$", you want to get any number in the set R where the size of R is \$r = d - t + 1\$. (This could be the numbers 4, 17, 23, etc., but the probability is the same.) In the example above, \$r = 20 - 10 + 1= 11\$ (the number of possible rolls that are 10 or higher on a d20). Then \$t = d - r +1\$, and we can substitue that into your formula as \$\frac{d+1-(d-r+1)}{d+2-(d-r+1)}\$, which simplifies to just \$\frac{r}{r+1}\$.
Feb 3 at 9:04 history edited Eddymage CC BY-SA 4.0
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Feb 2 at 16:46 history edited Eddymage CC BY-SA 4.0
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Feb 2 at 15:41 history edited Eddymage CC BY-SA 4.0
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Feb 2 at 15:21 history edited Eddymage CC BY-SA 4.0
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Feb 2 at 14:21 vote accept András
Feb 2 at 14:05 history edited Eddymage CC BY-SA 4.0
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Feb 2 at 13:56 history edited Eddymage CC BY-SA 4.0
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Feb 2 at 13:40 history edited Eddymage CC BY-SA 4.0
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Feb 2 at 13:32 history answered Eddymage CC BY-SA 4.0