91Ó°ÊÓ

Let the number of hours that Pham chooses to work at the daycare center be w, that at the bookstore be x, that at the café be y, and that at the garage be z. Her income equation will be

The daycare center will provide Pham with the least wages, which is $11.5 per hour for any number of hours she chooses to work. The marginal benefit in working at the bookstore rather than the daycare center is $0.5. Therefore, she will not work in the daycare center.

As the wages per hour at the café for 6 hours provide a maximum marginal benefit, she will work at the café for 6 hours. The next highest paying job is at the garage. So, she will work at the garage for 5 hours. She can work at the college bookstore for the remaining hours.

As Pham has a total of 15 hours to work, she will work at the café for 6 hours and at the garage for 5 hours. Then, she will work at the bookstore for the remaining hours. The hours can be estimated as follows.

$$\begin{gathered} 15=x+y+z \\ 15=x+6+5 \\ 15=x+11 \\ x=15-11=4 \end{gathered}$$

Therefore, Pham will work for 4 hours at the college bookstore.