Answer: [tex]\boldsymbol{37.5} < \mu < \boldsymbol{44.5}[/tex]
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Explanation:
At a 90% confidence level, the z critical value is roughly z = 1.645. Use a reference table or a calculator to determine this. Despite not knowing what sigma is, we can use a z interval here because n > 30. If the sample size n was smaller than 30, then we'd have to use a T distribution instead.
We'll plug that z value, along with the other given values, into the formula for the lower boundary L of the confidence interval.
[tex]L = \text{lower boundary}\\\\L = \overline{x} - z*\frac{s}{\sqrt{n}}\\\\L = 41 - 1.645*\frac{12}{\sqrt{31}}\\\\L = 41 - 3.545\\\\L = 37.455\\\\L = 37.5\\\\[/tex]
This value is approximate.
Do the same for the upper boundary as well
[tex]U = \text{upper boundary}\\\\U = \overline{x} + z*\frac{s}{\sqrt{n}}\\\\U = 41 + 1.645*\frac{12}{\sqrt{31}}\\\\U = 41 + 3.545\\\\U = 44.545\\\\U = 44.5\\\\[/tex]
This value is also approximate.
We have 90% confidence that the population mean [tex]\mu[/tex] (greek letter mu) is somewhere between L = 37.5 and U = 44.5
Therefore, we would write the 90% confidence interval as [tex]\boldsymbol{37.5} < \mu < \boldsymbol{44.5}[/tex]
This is the same as writing the confidence interval in the form [tex](\boldsymbol{37.5} , \boldsymbol{44.5})[/tex]. Both forms are two different ways to say the same thing. The first form being [tex]L < \mu < U[/tex] while the second form is [tex](L,U)[/tex].
Side note: The value 3.545 calculated earlier is the approximate margin of error.
