4. A decision at the margin Rosa is a hard-working college junior. One Saturday, she decides to work nonstop until she has answered 150 practice problems for her physics 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

Time total problems answered
8:00 AM 0
9:00 AM 60
10:00 AM 105
11:00 AM 135
Noon 150
Use the table to answer the following questions.

The marginal, or additional, gain from Rosa’s first hour of work, from 8:00 AM to 9:00 AM, is 60 problems.

The marginal gain from Rosa’s third hour of work, from 10:00 AM to 11:00 AM, is 30 problems.

Later, the teaching assistant in Rosa’s physics course gives her some advice. “Based on past experience,” the teaching assistant says, “working on 52.5 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?

A.1 hour working on problems, 3 hours reading
B. 2 hours working on problems, 2 hours reading
C. 3 hours working on problems, 1-hour reading
D.4 hours working on problems, 0 hours reading