Question

Suppose a geyser has a mean time between eruptions of 72 minutes. Let the interval of time between the eruptions be normally distributed with standard deviation 23 minutes. Complete parts ​(a) through ​(e) below.
​(a) What is the probability that a randomly selected time interval between eruptions is longer than 82​minutes? The probability that a randomly selected time interval is longer than 82 minutes is approximately nothing. ​(Round to four decimal places as​ needed.)
​(b) What is the probability that a random sample of 13 time intervals between eruptions has a mean longer than 82 ​minutes? The probability that the mean of a random sample of 13 time intervals is more than 82 minutes is approximately nothing. ​(Round to four decimal places as​ needed.)​
(c) What is the probability that a random sample of 34 time intervals between eruptions has a mean longer than 82 ​minutes? The probability that the mean of a random sample of 34 time intervals is more than 82 minutes is approximately nothing. ​(Round to four decimal places as​ needed.) ​
(d) What effect does increasing the sample size have on the​ probability? Provide an explanation for this result. Fill in the blanks below. If the population mean is less than 82 ​minutes, then the probability that the sample mean of the time between eruptions is greater than 82 minutes ▼ increases decreases because the variability in the sample mean ▼ decreases increases as the sample size ▼ decreases. increases. ​
(e) What might you conclude if a random sample of 34 time intervals between eruptions has a mean longer than 82 ​minutes? Select all that apply.
A. The population mean may be less than 72.
B. The population mean must be more than 72​, since the probability is so low.
C. The population mean cannot be 72​, since the probability is so low.
D. The population mean is 72​, and this is just a rare sampling.
E. The population mean may be greater than 72.
F. The population mean is 72​, and this is an example of a typical sampling result.
G. The population mean must be less than 72​, since the probability is so low.

Answer 1

Answer:

(a) The probability that a randomly selected time interval between eruptions is longer than 82 ​minutes is 0.3336.

(b) The probability that a random sample of 13-time intervals between eruptions has a mean longer than 82 ​minutes is 0.0582.

(c) The probability that a random sample of 34 time intervals between eruptions has a mean longer than 82 ​minutes is 0.0055.

(d) Due to an increase in the sample size, the probability that the sample mean of the time between eruptions is greater than 82 minutes decreases because the variability in the sample mean decreases as the sample size increases.

(e) The population mean must be more than 72​, since the probability is so low.

Step-by-step explanation:

We are given that a geyser has a mean time between eruptions of 72 minutes.

Also, the interval of time between the eruptions be normally distributed with a standard deviation of 23 minutes.

(a) Let X = the interval of time between the eruptions

So, X ~ N($\mu=72, \sigma^{2} =23^{2}$)

The z-score probability distribution for the normal distribution is given by;

                            Z  =  $\frac{X-\mu}{\sigma}$  ~ N(0,1)

where, $\mu$ = population mean time = 72 minutes

           $\sigma$ = standard deviation = 23 minutes

Now, the probability that a randomly selected time interval between eruptions is longer than 82 ​minutes is given by = P(X > 82 min)

       P(X > 82 min) = P( $\frac{X-\mu}{\sigma}$ > $\frac{82-72}{23}$ ) = P(Z > 0.43) = 1 - P(Z $\leq$ 0.43)

                                                           = 1 - 0.6664 = 0.3336

The above probability is calculated by looking at the value of x = 0.43 in the z table which has an area of 0.6664.

(b) Let $\bar X$ = sample mean time between the eruptions

The z-score probability distribution for the sample mean is given by;

                            Z  =  $\frac{\bar X-\mu}{\frac{\sigma}{\sqrt{n} } }$  ~ N(0,1)

where, $\mu$ = population mean time = 72 minutes

           $\sigma$ = standard deviation = 23 minutes

           n = sample of time intervals = 13

Now, the probability that a random sample of 13 time intervals between eruptions has a mean longer than 82 ​minutes is given by = P($\bar X$ > 82 min)

       P($\bar X$ > 82 min) = P( $\frac{\bar X-\mu}{\frac{\sigma}{\sqrt{n} } }$ > $\frac{82-72}{\frac{23}{\sqrt{13} } }$ ) = P(Z > 1.57) = 1 - P(Z $\leq$ 1.57)

                                                           = 1 - 0.9418 = 0.0582

The above probability is calculated by looking at the value of x = 1.57 in the z table which has an area of 0.9418.

(c) Let $\bar X$ = sample mean time between the eruptions

The z-score probability distribution for the sample mean is given by;

                            Z  =  $\frac{\bar X-\mu}{\frac{\sigma}{\sqrt{n} } }$  ~ N(0,1)

where, $\mu$ = population mean time = 72 minutes

           $\sigma$ = standard deviation = 23 minutes

           n = sample of time intervals = 34

Now, the probability that a random sample of 34 time intervals between eruptions has a mean longer than 82 ​minutes is given by = P($\bar X$ > 82 min)

       P($\bar X$ > 82 min) = P( $\frac{\bar X-\mu}{\frac{\sigma}{\sqrt{n} } }$ > $\frac{82-72}{\frac{23}{\sqrt{34} } }$ ) = P(Z > 2.54) = 1 - P(Z $\leq$ 2.54)

                                                           = 1 - 0.9945 = 0.0055

The above probability is calculated by looking at the value of x = 2.54 in the z table which has an area of 0.9945.

(d) Due to an increase in the sample size, the probability that the sample mean of the time between eruptions is greater than 82 minutes decreases because the variability in the sample mean decreases as the sample size increases.

(e) If a random sample of 34-time intervals between eruptions has a mean longer than 82 ​minutes, then we conclude that the population mean must be more than 72​, since the probability is so low.