A meteorologist was interested in the average speed of a thunderstorm in his area.

A meteorologist was interested in the average speed of a thunderstorm in his area. He sampled 13 thunderstorms and found that the average speed at which they traveled across the area was 15 miles per hour with a sample standard deviation of 1.7 miles per hour.

Assuming the speed of a thunderstorm is approximately normal, construct a 99% confidence interval for the true average speed of a thunderstorm in his area.

The answer I got was 13.8 and 16.2 but its incorrect an i put it in my Ti-84 in ZInterval.