Sprazz math > Reednut math
Perhaps I should have been less simplistic, but the question indicated that a rough estimate was required. Without setting a percentage requirement for getting the item, your formula doesn't provide an answer. Perhaps one way would be to use your formula with a 99 percent requirement. Also, you have chosen a rare (under 5k listed in the statistics vs 75k for the enforcers) ogre type, so I will use the .01 percent drop rate from the others.
Quote:
1-(1-averagedroprateforthemob)^(numberofmobskilled)=globaldroprate
If we want a 99 percent chance of getting the drop, then the numbers should look like this:
1-(1-0.0001)^x=0.99
Since I have Excel but no advanced math books nearby, I will iterate on the number of ogres until I achieve a 99 percent chance of the item dropping. If I am correct, then you would need to kill 46000 ogres. Multiplying by 20 seconds (per ogre kill) and dividing by 3600 to get the answer in hours, it will take
255.5 hours of grinding ogres to have a 99 percent chance of getting your plan. Please correct me if I missed something.
For reference, you would have only a 63.2 percent chance of getting the item after the 10000 ogres I mentioned in my first response. (Out of school too long FTL)