Hey I'm confused about how to do this kind of problem. I can't figure out how to find the standard deviation.
There are on average 4 tetanus cases reported in the US each month. What is the probability that at most 3 cases will be reported in any given month? What is the probability that 2 cases will be reported? What is the probability that 10 or more cases will be reported? What is the standard deviation of the number of cases per month?
You have to assume that the number of tetanus cases reported follows a Poisson distribution with $\lambda=4$.
Now using the Poisson distribution you can find out the given probabilities easily, and also the standard deviation.