In 2015, the average duration of long-distance telephone calls from a certain town was 3.9 minutes. A telephone company wants to perform a test to determine whether this average duration of long-distance calls has changed. 50 calls, originating from the town, was randomly selected and the following summary minutes.

= 205 ()² = 56.43

a) Calculate the sample mean, .

b) Calculate the sample standard deviation.

c) Using the p value approach at the 1% level of significance, test whether the mean duration of long-distance calls from the town had increased.

d) Construct the 99% confidence interval for the population mean duration of the long-distance calls from the town.