Failsafe Textiles employs highly skilled maintenance workers who are responsible for repairing the numerous industrial robots used in its manufacturing process. Workers operate independently and can repair one robot every 5 hours, on average. An average of one robot fails every 4 hours. Each down robot costs the company $220 per hour lost in manufacturing. Each maintenance worker costs the company $10 per hour in salary, benefits, and equipment. Assume for simplicity that the coefficient of variation for the inter-arrival times and processing times is 1?

1. How many workers should the manager have in order to guarantee that a failed robot could be returned to operational state within 7 hours - on average?