Lesson 5

In this lesson, students are introduced to conventions for using signed numbers to represent money spent and received, as well as inventory gained and lost. While money contexts can be represented without signed numbers, there are many situations that are more efficiently modeled by signed numbers. For example, if a person has \$50 in the bank and writes a \$20 check, we can represent the balance as \(50-20\). If they had written an \$80 check, we can still write the balance as \(50-80\), as long as we have adopted the convention that negative numbers represent what the person owes the bank (and assuming the bank allows overdrafts). Since students do not operate on signed numbers in this grade, this lesson is simply an introduction to the convention of using signed numbers to represent a change in money or a change in inventory, an important convention in modeling financial situations with mathematics (MP4). In a later lesson, students will be introduced to the idea of an account balance. In grade 7, students will study addition and subtraction of signed numbers and apply those concepts in accounting situations.