The formula for the confidence interval is;
Where:
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Given:
The mean is 25.4, the sample size n is 89, the standard deviation s is 2.4.
a. For the 95% confidence interval, the z-score for a 95% interval is 1.96.
Therefore, the 95% confidence interval is;
![\begin{gathered} CI=25.4-1.96\frac{2.4}{\sqrt[]{89}}<\operatorname{mean}<25.4+1.96\frac{2.4}{\sqrt[]{89}} \\ CI=24.90<\operatorname{mean}<25.90 \end{gathered}]()
Answer: (24.90, 25.90)
b. For the 90% confidence interval, the z-score for a 90% interval is 1.645.
Therefore, the 90% confidence interval is;
![\begin{gathered} CI=25.4-1.645\frac{2.4}{\sqrt[]{89}}<\operatorname{mean}<25.4+1.645\frac{2.4}{\sqrt[]{89}} \\ CI=24.98<\operatorname{mean}<25.82 \end{gathered}]()
Answer: (24.98, 25.82)
c. For the 99% confidence interval, the z-score for a 99% interval is 2.576.
Therefore, the 99% confidence interval is;
![\begin{gathered} CI=25.4-2.576\frac{2.4}{\sqrt[]{89}}<\operatorname{mean}<25.4+2.576\frac{2.4}{\sqrt[]{89}} \\ CI=24.74<\operatorname{mean}<26.06 \end{gathered}]()
Answer: (24.74, 26.06)