Answers and explanation:
1) In this case, the scarce resource is the time because is the factor that as a working-studying student, the student will not have much of it. Scarcity defines the basic economic problem of individuals with unlimited wants and needs only counting with limited resources to fulfill them.
2) The decision of how many hours to work will be taken with a trade-off. It implies sacrificing part of the satisfaction of a need to satisfy more another and combined generate a greater joy. Thus, the hours chosen will depend on how much free time and classes the student is willing to sacrifice for work and earn a salary.