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 callouts in one hour.

I understand the formula for poisson, which is:

where is the mean. But I substitute =7, r=5 in and get the wrong answer. I then substiture 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 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?