The time to failure of fluorescent lights in a large office building is exponentially distributed with a failure rate of XXXXXXXXXXhr. How many spare tubes must the building custodian maintain to have...

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.