Imagine you are tutoring a new student who is working on a homework assignment about solving word problems that require two-digit subtraction. The first question on the assignment is:

“Jessica wants to buy a book that costs $15 and she has $32 in her wallet. After she buys the book, how much money will she have left?”

Your student looks at the problem and looks back at you, terrified. He has no idea what to do. Clearly he hasn’t yet mastered the ability to solve word problems with two-digit subtraction. But what exactly is the problem? …