Adesign engineer wants to construct a sample mean chart for controlling the service life of a halogen headlamp his company produces. he knows from numerous previous samples that this service life is normally distributed with a mean of 500 hours and a standard deviation of 20 hours. on three recent production batches, he tested service life on random samples of four headlamps, with the following results: what is the standard deviation of the sampling distribution of sample means for whenever service life is in control? sample service life (hours. 1 495 500 505 500 2 525 515 505 515 3 470 480 460 470 select one: a. 10 hours b. 6.67 hours c. 11.55 hours d. 5 hours e. 20 hours