number of called numbers can go anywhere from 8 to 75. this is the expression
http://www.wolframalpha.com/input/?i...++),+n%3D8..75
see that sum is 1 so it must be correct.
if you want the probability of getting it called within first 15 numbers, that is the sum from above link from 8 to 15. that would be a probability of 0.00003814%, so it is very unlikely.
for the first 20 it's still very small, 0.0007467%
you need to increase it all the way to 60 to get 15.17%, which I have to admit is an somewhat unexpected result. but it should be correct since it did sum to 1 for n in interval [8,75].
see the partial sums chart to see how the probability grows.
