# Math Help - Poisson distribution question.

1. ## Poisson distribution question.

The question is about a mechanic.

To summarise, he gets 7 calls per hour. The question wants to know what the probability is of him getting $<5$ callouts in one hour.
I understand the formula for poisson, which is:
$\frac{e^{-\lambda}\times\lambda^r}{r!}$ where $\lambda$is the mean. But I substitute $\lambda$=7, r=5 in and get the wrong answer. I then substiture $\lambda = \frac{7}{60}$ and r=5 but this is still incorrect.

The answer is 0.173 to 3 d.p.

I have the poisson cumulative frequency tables to save you some working.

Edit: I now understand what $\lambda$ represents, and know how to get the correct answer from my cumulative frequency table. I just don't understand what the r! represents?