Final answer:
The degrees of freedom (df) for this study is equal to the sample size minus 1. The appropriate t value for a two-tailed 95 percent confidence interval can be found in the t-distribution table. The 95 percent confidence interval can be calculated by taking the sample mean and adding/subtracting the margin of error.
Step-by-step explanation:
The number of degrees of freedom (df) for this study is equal to the sample size minus 1. In this case, the sample size is not specified in the question, so we cannot determine the exact value of df.
For a two-tailed 95 percent confidence interval, the appropriate t value to use in the formula can be found by looking up the critical value in the t-distribution table. The critical value for a two-tailed 95 percent confidence interval with a given degree of freedom (df) can also be calculated using software or a calculator.
The 95 percent confidence interval can be calculated by taking the sample mean and adding/subtracting the margin of error. The margin of error is equal to the critical value multiplied by the standard error. The standard error is the standard deviation divided by the square root of the sample size.
The 99 percent confidence interval can be calculated in the same way as the 95 percent confidence interval, using the appropriate critical value for a two-tailed 99 percent confidence interval. Round the endpoints of the confidence intervals to two decimal places.
If the sample size was 30 instead of 20, the 95 percent confidence interval would have a smaller margin of error, resulting in a narrower interval.