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.

Safety regulations require that no more than 20% of the bulbs be burned out at any given time. To be on the safe side, the company would like to change them so that no more than 15% burn out before they are changed. After how many hours of use should the bulbs be changed in order to meet this goal?