Posted by **Fran** on Thursday, March 4, 2010 at 8:56am.

A disk drive manufacturer has determined that his product has a “mean time to failure” of 10,000 hours, with a standard deviation of 500 hours. Due to the many factors involved, it is assumed that the time to failure is normally distributed. A network operations center purchases 200 of these disk drives and places them into service on the same day. They run continuously, 24/7. After how many hours can we reasonably expect that 99% of the drives are still running? At the moment the second drive (out of 200) fails, how much time do we have until we expect that 30% of the original 200 drives will have failed? Please explain your reasoning in obtaining the answer.