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 $\displaystyle <5$ callouts in one hour.

I understand the formula for poisson, which is:

$\displaystyle \frac{e^{-\lambda}\times\lambda^r}{r!} $ where$\displaystyle \lambda $is the mean. But I substitute $\displaystyle \lambda $=7, r=5 in and get the wrong answer. I then substiture $\displaystyle \lambda = \frac{7}{60}$ 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.

Please help?

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

I don't expect a huge, half a page formula, because I wouldn't want to waste any one's time, but how would I get the answer without the Poisson cumulative frequency diagram?