# Find the probability that X lies within one standard deviation of the mean.

The random variable X has a Poisson distribution with mean $μ=5.55$. Find the probability that X lies within one standard deviation of the mean.

Using my calculator, I found $P(3.19<X<7.9)$ where $3.19 = 5.55 - \sqrt{5.55}$ and $7.9 = 5.55 + \sqrt{5.55}$ and got $0.683$.

• How did you get $0.683$? – David Oct 31 '16 at 2:04
• normalcdf($5.55 - \sqrt{5.55}$, $5.55 + \sqrt{5.55}$, $5.55$, $\sqrt{5.55}$) – StopReadingThisUsername Oct 31 '16 at 2:21