The formula for the confidence interval is;
Where:
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;
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;
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;
Answer: (24.74, 26.06)