Use the standard normal distribution or the t-distribution

Question

 Use the standard normal distribution or the t-distribution to construct a 95% confidence

interval for the population mean. Justify your decision. If neither distribution can be used, explain why. Interpret the results.
In a random sample of 21 mortgage institutions, the mean interest rate was 3.62% and the standard deviation was 0.44%. Assume the interest rates are normally distributed.

 
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