Karen is a trusted employee whose productivity decreases as she works longer and longer hours each day. After careful observation of her job performance, her manager prepared the following chart.

Daily number of hours worked by Karen / Total number of units of work completed
1/100
2/190
3/270
4/340
5/400
6/450
7/480
8/500

Karen's total cost to the company is $11 per hour. Each unit of work completed is worth $0.21 to the company. Ignoring all other possibilities and considerations, how many hours per day should the company hire Karen?