A decision at the margin

Becky is a hard$-$working college freshman. One Tuesday, she decides to work nonstop until she has answered $100$ practice problems for her math 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.

$\backslash$table$[[$Time$,$Total Problems Answered$],[8$:$00$ AM$,0],[9$:$00$ AM$,40],[10$:$00$ AM$,70],[11$:$00$ AM$,90],[$Noon$,100]]$

Use the table to answer the following questions.

The marginal, or additional, benefit from Becky's first hour of work, from $8$:$00$ AM to $9$:$00$ AM$,$ is $$ problems.

The marginal benefit from Becky's third hour of work, from $10$:$00$ AM to $11$:$00$ AM$,$ is $$ problems.

Later, the teaching assistant in Becky's math course gives her some advice. "Based on past experience," the teaching assistant says, "working on $15$ 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?

$0$ hours working on problems, $4$ hours 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