statistics
posted by Anonymous .
According to a study of 90 truckers, a trucker drives, on average, 540 miles per day. If the standard deviation of the miles driven per day for the population of truckers is 40, find the 99% confidence interval of the mean number of miles driven per day by all truckers.

You need to find SD that includes ± 49.5% (.495) of the cases from the mean on both sides (.495 + .495 = .99).
99% confidence interval = mean ± 2.585 SD