Marck wrote (from Never-Never Land):
>
> Hello, I would need help to solve the following question.
>
> At a Web server every 30 seconds arrive in average 3 requests. I want
> to calculate the probability of waiting for more than 15 minutes so
> that 80 requests arrive.
A valuable technique for questions of this type involves the "logically
equivalent problem", interchanging the parameter and the variable in a
cumulative process. (It works because cumulative values are monotone and
hence invertible, which may give you a feel for when to try it and when
not to.)
Thus,
waiting for more than 20 days for 17 clients to sign on
is logically the same as
getting strictly fewer than 17 clients in 20 days.
which is a simple Poisson-type problem.
Now, "You know my methods, Watson: apply them." Not that this is
specifically my method, you understand...
-Robert Dawson
.
.
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