We use the t distribution for the statistical inference of the population mean when the underlying population

Question:

We use the t for the statistical inference of the population mean when the underlying population standard deviation is not known. Under the assumption that the population is normally distributed, find tα/2,df for the following scenarios.
a. A 90% confidence level and a sample of 28 observations.
b. A 95% confidence level and a sample of 28 observations.
c. A 90% confidence level and a sample of 15 observations.
d. A 95% confidence level and a sample of 15 observations.

Distribution
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