4. Optica Ltd. makes prescription glasses according to orders received randomly from customers. Each worker is specialized in certain types of glasses. The company has been experiencing unusual delays in the processing of bifocal prescriptions. The worker in charge of bifocals receives 30 orders per 8-hour working day. The time to completea prescription is approximately normally distributed, with a mean of 12 minutes and a standard deviation of 3 minutes. After spending between 2 and 4 minutes, uniformly distributed, to inspect the glasses, the worker can start on a new prescription. The sum of these independent times is the service
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