A firm employs Pam to assemble personal computers. Pam can assemble 1 computer if she works 1 hour, 4 computers in 2 hours, 8 computers in 3 hours, 10 computers in 4 hours, and 11 computers in 5 hours. Each computer consists of a motherboard that costs $200, a hard drive that costs $100, a case that costs $20, a monitor that costs $200, a keyboard that costs $60 and a mouse that costs $20. The cost of employing Pam is $40 per hour.
The firm sells each computer for $640. How many hours should the firm employ Pam to maximize its benefit from her employment?