Need IMMEDIATE help now with this question: (Not sure how to tackle this)

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

Daily Number of Hours Worked by Karen / Total Numbers of Work Units Completed

1/100

2/190

3/270

4/340

5/400

6/450

7/480

8/500

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