In this lesson, students solve "add to with change unknown" word problems and discuss the connection between addition and subtraction. For example, when solving the problem "There were 5 books on a shelf. A student came and put some more books on the shelf. Now there are 7 books on the shelf. How many books did the student bring?" students write the number sentences 5 + ___ = 7 and 7 - 5 = ___ and use strategies like counting on to solve.