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6. In 2015, the average duration of long-distance telephone calls from a certain town was 9.4

minutes. A telephone company wants to perform a test to determine whether this average

duration of long-distance calls has changed. Twenty calls, originating from the town, were

randomly selected and the mean duration was 10.2 minutes with standard deviation of 4.8

minutes.

a) Using a 1% level of significance, test whether the mean duration of long-distance calls

from the town had increased.

[6]

b) Construct the 99% confidence interval for the population mean duration of the long-

distance calls from the town.

[4]

Fig: 1