A large industrial complex would like to design a schedule for changing its lightbulbs. For proper lighting, it is essential that the bulbs be changed prior to burning out. In order to develop the schedule, the company models the bulbs’ useful lifetime using a normal distribution. They know from past experience that on average bulbs will last 1700 hours before burning out and they estimate that the standard deviation to be 150 hours. One proposal is to change the bulbs every 1620 hours (about every 90 days). If that plan is used, what percentage of lightbulbs will burn out before they are changed?

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If XLifetime of a bulb in hrs The... View the full answer