A design engineer wants to construct a sample mean chart for controlling the service life of one of the types of light bulbs his company produces. The engineer 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, the engineer tested service life on random samples of four bulbs, with these results: Sample 1 2 3 495 525 470 Service Life (hours) 500 505 515 505 480 460 500 515 470 If upper and lower control limits of 520 and 480 hours are used, on what samples) (if any) does service life appear to be out of control