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	Service Life (hours)
1	495	500	505	500
2	525	515	505	515
3	470	480	460	470

What is the sample mean service life for sample 2?

Multiple Choice

515 hours

500 hours

525 hours

495 hours

460 hours