Paper-and-Pencil Subtraction

Explain to students that it is very important for them to develop the habit of checking their work. The nice thing about subtraction is that we can check it with addition. Sometimes when we look back over calculations, we make the same mistake we did the first time. But by checking subtraction with a different operation—addition—we can sometimes pick up errors we would not otherwise catch.

Begin by displaying a subtraction problem with an incorrect answer, as in Figure 5.

Read the Checking Your Work with Addition section of the Paper-and-Pencil Subtraction pages in the Student Guide as a class. Then ask them to check the problems in Question 4 and determine which are correct and which are incorrect. (The second problem is incorrect.)

Assign Questions 1–2 on the Checking with Addition page in the Student Activity Book. In Question 1, students use addition to check someone else's work. In Question 2, they subtract and then use addition to check their own work.

Assure students that it is not a weakness to check their work but a strength. Even the best mathematicians make mistakes in their calculations. One thing that makes them “the best” is that they have the habit of checking their work so they can find and correct their mistakes.

After students are comfortable with checking, they might prefer to not write down the sum when they check. They can check using mental math to add. For example, after solving the problem in Figure 6, they can add the two bottom numbers in their heads to see if they get the number at the top that they started with.