A factory makes propeller drive shafts for ships. A quality assurance engineer at the factory needs to estimate the true mean le
ngth of the shafts. She randomly selects four drive shafts made at the factory, measures their lengths, and finds their sample mean to be 1000 mm. The lengths are known to have a normal distribution with a standard deviation is 2 mm. Calculate a 95% confidence interval for the true mean length of the shafts.