A large manufacturing plant uses a certain brand of light bulbs that has a mean time to failure (MTTF) of 3600 hours with a standard deviation of 160 hours. Good lighting in the plant is critical to safety, and the plant manager determines that no more than 10% of the bulbs should fail while being used. He institutes a policy to replace all the light bulbs simultaneously according to a set schedule, rather than to replace individual bulbs as they burn out. After how manyhours should all the bulbs be replaced such that no more than 10% of the bulbs will fail while being used? (You may assume that the variations in MTTF are random.)