A design 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 these results: Sample Hours 1 497 489 478 490 2 500 491 498 518 3 509 513 500 494 What is the sample mean service life for sample 2? ((Keep two decimal places)