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The annual rainfall in a city is approximately normally distributed with a mean of 68 inches and a standard deviation of 3 inches. What is the probability that the annual rainfall in this city in a randomly selected year is between 68 inches and 76 inches? Round your answer to four decimal places.

A. 0.5038
B. 0.4962
C. 0.5302
D. 0.0038
E. 0.7052

Answer :

We are given that the annual rainfall, [tex]$X$[/tex], is normally distributed with a mean [tex]$$\mu = 68$$[/tex] inches and a standard deviation [tex]$$\sigma = 3$$[/tex] inches. We wish to find the probability that the rainfall is between 68 inches and 76 inches, i.e., we want to calculate

[tex]$$
P(68 \le X \le 76).
$$[/tex]

Step 1. Standardization

To find this probability we first convert the values to [tex]$z$[/tex]-scores using the standardization formula

[tex]$$
z = \frac{x - \mu}{\sigma}.
$$[/tex]

For the lower bound, when [tex]$x = 68$[/tex], the [tex]$z$[/tex]-score is

[tex]$$
z_{\text{lower}} = \frac{68 - 68}{3} = 0.
$$[/tex]

For the upper bound, when [tex]$x = 76$[/tex], the [tex]$z$[/tex]-score is

[tex]$$
z_{\text{upper}} = \frac{76 - 68}{3} = \frac{8}{3} \approx 2.6667.
$$[/tex]

Step 2. Finding the Corresponding Probabilities

Under the standard normal distribution, let [tex]$\Phi(z)$[/tex] be the cumulative distribution function (CDF) which gives the probability that a standard normal variable is less than or equal to [tex]$z$[/tex].

- For [tex]$z = 0$[/tex], we have

[tex]$$
\Phi(0) = 0.5.
$$[/tex]

- For [tex]$z \approx 2.6667$[/tex], the cumulative probability is approximately

[tex]$$
\Phi(2.6667) \approx 0.99617.
$$[/tex]

Step 3. Calculating the Desired Probability

The probability that [tex]$X$[/tex] is between 68 and 76 inches corresponds to the probability that [tex]$Z$[/tex] is between 0 and 2.6667. This can be calculated as

[tex]$$
P(0 \le Z \le 2.6667) = \Phi(2.6667) - \Phi(0).
$$[/tex]

Substituting the values we found,

[tex]$$
P(68 \le X \le 76) = 0.99617 - 0.5 \approx 0.49617.
$$[/tex]

Rounding to four decimal places, we obtain

[tex]$$
P(68 \le X \le 76) \approx 0.4962.
$$[/tex]

Final Answer

The probability that the annual rainfall is between 68 inches and 76 inches is approximately [tex]$$0.4962.$$[/tex]

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