Possible Duplicate:
Birthday-coverage problem
An example of what I wish to do is the following: https://stackoverflow.com/questions/4681913/substr-md5-collision/4785456#4785456
How would I calculate how many people would be required, as in the link above, to reach 50% or 0.001% or n% probability of collision exactly?
I am able to calculate the likelyhood of a collision in say a hash, with
$1-e^\frac{-n^2}{(2*10^6)}$
10^6 being six numerical digits from zero to nine. However, I would have to guess a lot of times before I got the exact number of people it would take to reach exactly 50%, which may be a fraction (i.e. 20.2 people)
How would I be able to find this?