Part (a) Mary is planning to do two part-time jobs, one in the retail store ABC and the other in the restaurant LMNO, to earn tuition. She decides to earn at least $120 per week. In ABC, she can work 5 to 12 hours a week, and in LMNO, she can work 4 to 10 hours a week. The hourly wages of ABC and LMNO are $6 per hour and $8 per hour, respectively. When deciding how long to work in each place, Mary hopes to make a decision based on work stress. According to reviews on the Internet, Mary estimates that the stress levels of ABC and LMNO are 1 and 2 for each hour of working, respectively (stress levels are between 1 and 5; a large value means a high work stress which may cause work and life imbalance). Since stress accumulates over time, she assumes that the total stress of working in any place is proportional to the number of hours she works in that place.

1. How many hours should Mary work in each place per week?

2. State verbally the objective, constraints and decision variables.

3. Then formulate the problem as an LP model.

4. After that, solve it using the graphical solution procedure.

Part (b) The estimated stress level for working at ABC was obtained from a few, not many, reviews on the Internet, so the estimate is rough. If the true stress level is believed to be in the range of 1 to 1.5 and its exact value cannot be known, explain whether Mary is able to determine the best time allocation.