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Chance to get desired drop

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If you are looking for a very special drop, and are wondering about how many mobs it might take, or if you are grinding for an item and didn't get it, you might want to know what the chance of getting it was, and this formula might come in handy.

P = 1 - ((1 - D) ^ X)

P = % chance to get desired drop.
D = drop rate of desired item. (x of x-kills), this means that a 10% drop chance is equal to 0.10.
X = the amount of mobs you plan to kill.
Expressions in parentheses are to be calculated first.

An example:
You want to farm BRD for the Inv misc note 01 [Formula: Enchant Weapon - Fiery Weapon], you searched Thottbot, and noticed that the drop rate was about 14%, now you wonder what's the chance you'll get it in 5 runs?
Easy: P = 1 - ((1 - 0.14) ^ 5)
After doing this little formula, you'll end up knowing that, after these five runs, then there is a 53% chance that you got the recipe. Not 70% as many might think (14 * 5).

This formula is usable it other ways as well. If you want to make sure you have at least an 80% chance of getting a desired item, then just isolate X, and calculate that. This will give how many kills it'll take to get a 80% chance to get the desired item, and thereby you'll know if you're going to use an hour or three days on grinding for it.

An example:
You want to farm BRD for the same Formula: Enchant Weapon - Fiery Weapon recipe (above), with a 14% drop rate. You wonder how many kills (runs) it'll take to get a 80% chance to get the recipe?
Easy: 0.80 = 1 - ((1 - 0.14) ^ x)
After doing this little formula, you'll end up knowing that, to get a 80% chance to get the recipe you'll have to do 10.7 runs (11 of course, since you can't do a 0.7 kill).
Kills compared with % chance to get drop
Kills drop chance
The graph above is based on the numbers in the table below.
Kills  % chance - Kills  % chance
1 14% - 9 74.3%
2 26% - 10 77.9%
3 36.4% - 11 81%
4 45.3% - 12 83.6%
5 53% - 13 85.9%
6 59.5% - 14 87.9%
7 65.2% - 15 89.6%
8 70.1% - 16 91%

This table shows the % chance to get the Formula: Enchant Weapon - Fiery Weapon recipe from BRD, kills compared with % chance. It's easy to see that the % chance increases less for each additional kill.

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