Question Completion Status QUESTION 2 A design engineer wants to construct a sample mean chart for controlling the service life of a halogen headiamp 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 deration of 20 hours. On four recent production batches, he tested service life on random samples of four headlamps with these results. Sample Service Life in Hours 8:00am 10:00am 12:00pm 2:00pm a 495 500 SOS 500 b 500 516 505 515 C 505 505 460 474 d 500 515 474 531 (5 points) What is the grand mean service level? 460 Hours 495 Hours 500 Hours 510 Hours 525 Hours