Answer:
The 80% confidence interval for the mean usage of water is between 18.4 and 18.6 gallons per day.
Step-by-step explanation:
We have that to find our [tex]\alpha[/tex] level, that is the subtraction of 1 by the confidence interval divided by 2. So:
[tex]\alpha = \frac{1-0.8}{2} = 0.1[/tex]
Now, we have to find z in the Ztable as such z has a pvalue of [tex]1-\alpha[/tex].
So it is z with a pvalue of [tex]1-0.1 = 0.90[/tex], so [tex]z = 1.28[/tex]
Now, find the margin of error M as such
[tex]M = z*\frac{\sigma}{\sqrt{n}}[/tex]
In which [tex]\sigma[/tex] is the standard deviation of the population and n is the size of the sample.
[tex]M = 1.28*\frac{2.3}{\sqrt{717}} = 0.1[/tex]
The lower end of the interval is the sample mean subtracted by M. So it is 18.5 - 0.1 = 18.4 gallons per day.
The upper end of the interval is the sample mean added to M. So it is 18.5 + 0.1 = 18.6 gallons per day.
The 80% confidence interval for the mean usage of water is between 18.4 and 18.6 gallons per day.
