This is an advanced question, so just give it a try. Can you use indifference curves, and a budget line describing a choice between consumption/monetary income and leisure, to explain the results of the previous two questions? That is, how can it be that when wages increase from 25 to 30, hours worked INCREASE, but when wages then increase from 30 to 35, hours worked DECREASE? You may suppose that the typical graduate student has 40 hours of time for work or leisure in a week.