5. A decision at the margin Suppose that Darnell is a diligent hard-working second-year college student. One Wednesday, he decides to start the day by working through 300 practice problems to study for the econometrics class. Using a table to track his progress throughout the day, he notices that as the hours pass, it takes him longer to solve the problems. Time Total Problems Solved 7:00 AM 0 8:00 AM 120 9:00 AM 210 10:00 AM 270 11:00 AM 300 Use the table to answer the following questions. The marginal, or additional, gain from Riley's second hour spent working, from 8:00 AM to 9:00 AM, is problems. The marginal gain from Riley's fourth hour spent working, from 10:00 AM to 11:00 AM, is problems. The following week, Riley's econometrics faculty advisor gives her some advice. In all their years of teaching they claim to observe that working on 105 multiple choice questions boosts a student's test score by about the same amount as spending an hour reviewing lecture notes. For simplicity, assume students are able to review lecture notes at a constant pace during each hour spent studying.