# Solved (Free): The annual precipitation amounts in a certain mountain range are normally distributed with a mean of 91 inches

#### ByDr. Raju Chaudhari

Mar 9, 2021

The annual precipitation amounts in a certain mountain range are normally distributed with a mean of 91 inches, and a standard deviation of 14 inches. What is the probability that the mean annual precipitation during 49 randomly picked years will be at least 92.5 inches?

#### Solution

Let $X$ denote the annual precipitation amounts in a certain mountain range.

Given that population mean $\mu = 91$, population standard deviation $\sigma = 14$ and the sample size $n = 49$.

$X\sim N(91, 14^2)$. Thus $\overline{X}\sim N(91, 14^2/49)$.

The probability that the mean annual precipitation during 49 randomly picked years will be at least 92.5 inches is

 \begin{aligned} P(\overline{X}\geq 92.5) & =1- P(\overline{X} < 92.5)\\ & = 1- P\bigg(\frac{\overline{X}-\mu}{\sigma/\sqrt{n}} < \frac{92.5-91}{14/\sqrt{49}} \bigg)\\ & = 1-P\big(Z < 0.75 \big)\\ &= 1- 0.7734\\ & = 0.2266\\ \end{aligned}