The marginal, or additional, gain from kyoko's first hour of work, from 8: 00 am to 9: 00 am, is 80 problems.
the marginal gain from kyoko's third hour of work, from 10: 00 am to 11: 00 am, is 40 problems.
later, the teaching assistant in kyoko's chemistry course gives her some advice. "based on past experience," the teaching assistant says, "working on 30 problems raises a student's exam score by about the same amount as reading the textbook for 1 hour." for simplicity, assume students always cover the same number of pages during each hour they spend reading.
given this information, in order to use her 4 hours of study time to get the best exam score possible, how many hours should she have spent working on problems, and how many should she have spent reading?
- 3 hours working on problems, 1 hour reading

When Kyoko solves problems during 3 hours and reads for 1 hour, her learning utility is equivalent to 150 problems solved.

Explanation:

We are not given the information about what happens between 9:00 to 10:00 AM, so we can assume that their is no marginal gain from solving problems during that hour.

Kyoko will be able to solve 120 problems (= 80 + 40) during 8:00 to 11:00 AM. Apparently she will not be able to solve any other problems beyond 11:00 AM, so in order to maximize her learning utility she must find another activity that will help her raise her test scores.

She has 1 free hour and we know that reading for 1 hour is equal to solving 30 problems, so she could decide to do that (apparently is that or doing nothing).

When Kyoko solves problems during 3 hours and reads for 1 hour, her learning utility is equivalent to 150 problems solved.

answer; dividing net income by 1- t ;