The posted speed limit on a particular segment of an interstate highway is 65 mph. From past experience, the standard deviation of vehicle speed is 11 mph. To estimate the average speed of vehicles, the speeds of 60 vehicles are recorded and the average speed is estimated to be 69 mph. (a) Determine the 95% confidence interval for the mean vehicle speed. (b) Determine the 95% lower and upper confidence limits for the mean vehicle speed. (c) The confidence interval estimated in Part (a) needs to be reduced by 20%. How many additional measurements are needed? (d) The confidence level of the confidence interval estimated in Part (a) needs to be increased from 95% to 99%. How many additional measurements are required?