Last month, a large national banks average payment time was 33 days with a standard deviation of

Question:

Last month, a large national bank€™s average payment time was 33 days with a standard deviation of 4 days. This month, the average payment time was 33.5 days with a standard deviation of 4 days. They had 1,000 customers both months.

Build a 95% confidence interval on the difference between this month€™s and last month€™s payment times. Is there evidence that payment times are increasing?