The time to failure of fluorescent lights in a large office building is exponentially distributed with a failure rate of 0.03125/hr. How many spare tubes must the building custodian maintain to have at least a 0.95 probability of replacing all failures on a given day? Assume continuous 24-hour use of the lights. 4. A 60-watt outdoor lightbulb is advertised as having an average life (i. e., MTTF) of 1000 (operating) hours. However, experience has shown that it will also fail on demand an average of once every 120 cycles. A particular bulb is turned on once each evening for an average of 10 hr. If it is desired to have a reliability of 90 percent, what is its design life in days?