*4. Optica, Ltd., makes prescription glasses according to orders received from customers.

Each worker is specialized in certain types of glasses.The company has been experiencing unusual delays in the processing of bifocal and trifocal prescriptions. The worker in charge receives 30 orders per 8-hour day. The time to complete a prescription is 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. Determine the following:

(a) The percentage of time the worker is idle.

(b) The average backlog of bifocal and trifocal prescriptions in Optica.

(c) The average time until a prescription is filled.