Poisson

Patriots326 · #1 $Old$ December 17th, 2008, 05:38 AM

Cars arrive at a toll booth at a mean rate of 15 cars every half hour according to a poisson process. Find the probability that the toll collector will have to wait longer than 36.78 minutes before collecting the eleventh toll.
Please Help!!!

meymathis · #2 $Old$ December 17th, 2008, 03:26 PM

I will use the formulation of Poisson from wikipedia.

$\lambda$ is the expected number of occurrences over the particular interval of interest. So in our case the expected rate of occurrence is $15/30=0.5$ cars per minute. So the expected number of cars in 36.78 minutes is: $\lambda=15/30*36.78=18.4$

If $N$ is the number of cars that arrive in 36.78 minutes, then the question can be rephrased to: what is the probability that $N<11$

The PDF is:
$f(k; \lambda)=\frac{\lambda^k e^{-\lambda}}{k!}$

So

$\mathrm{P}(N<11) = \sum_{k=0}^{10}\frac{\lambda^k e^{-\lambda}}{k!}=\ldots$