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. 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 bulbs, with these results:

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

What value should be used as the mean of the sampling distribution of sample means?

Select one:

a.

470 hours

b.

495 hours

c.

525 hours

d.

515 hours

e.

None