A warehouse is lit by 1000 f luorescent tubes, each of which fails, on average, after 4250 hours of operation. It’s costly to send a technician to replace an individual tube when it fails, so the warehouse manager decides to replace all the tubes after 3000 hours of operation, whether they have failed or not. On average, how many failed tubes will be replaced? Assume an Exponential distribution for this “time to failure.”