Susan is a hard-working college junior. One Thursday, she decides to work nonstop until she has answered 50 practice problems for her economics course. She starts work at 8:00 AM and uses a table to keep track of her progress throughout the day. She notices that as she gets tired, it takes her longer to solve each problem.

Time	Total Problems Answered
8:00 AM	0
9:00 AM	20
10:00 AM	35
11:00 AM	45
Noon	50

Use the table to answer the following questions.

The marginal, or additional, gain from Susan's first hour of work, from 8:00 AM to 9:00 AM, is

problems.The marginal gain from Susan's third hour of work, from 10:00 AM to 11:00 AM, is

problems.

Later, the teaching assistant in Susan's economics course gives her some advice. "Based on past experience," the teaching assistant says, "working on 7.5 problems raises a student's 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 score possible, how many hours should she have spent working on problems, and how many should she have spent reading?

1 hour working on problems, 3 hours reading

2 hours working on problems, 2 hours reading

3 hours working on problems, 1 hour reading

4 hours working on problems, 0 hours reading