In 2002, the average duration of incoming telephone calls to a furniture store was 9.4 minutes. The manager wants to perform a test to determine whether this average duration of incoming calls has...

In 2002, the average duration of incoming telephone calls to a furniture store was 9.4 minutes. The manager wants to perform a test to determine whether this average duration of incoming calls has changed. Twenty calls to the store was randomly selected and the mean duration was 10.2 minutes with standard deviation 4.8 minutes. Assuming that the duration of incoming telephone calls to a furniture store is normally distributed.

 a) Using a 1% level of significance, test whether the mean duration of incoming calls to the store had changed.