Make Connections with Subtraction

Est. Class Sessions: 2

Developing the Lesson

Part 2: Common Mistakes in Subtracting

Upon completion of the Solve It Three Ways pages, ask:

Which is your favorite strategy to use? Why?
Is one strategy quicker or more efficient than another?
Is one strategy easier to understand?
Is there anything difficult or tricky about one of the methods? What is it?

Display the Fix It pages from the Student Activity Book. On this page, students are shown common errors made when subtracting. Direct students’ attention to the first problem in Question A. Have students discuss the problem and try to figure out what went wrong. Then briefly discuss the mistake as a class. Finally, ask students to solve the problem correctly. Repeat these steps for each of the problems. Discussion should focus on the following errors:

Question A: The student subtracted up (6 − 0 instead of 0 − 6).

Question B: The student regrouped when he didn’t need to.

Question C: The numbers are aligned differently and there is a
place value error.

Question D: A trade for 100 was made when only a trade for
10 was necessary.

Question E: When a ten was taken from 6 tens, the student
added 10 + 6 and recorded 16 instead of 5.

Question F: The student regrouped but forgot to add the existing
ones (10 + 3 = 13).

Write the following problem on the board and ask:

165 = 100 + 60 + 5
71 = 70 + 1
30 + 59 + 5 = 165

Ask:

Look at this solution to 165 − 71. What do you notice about the strategy? (Possible response: You took 70 from 100 and 1 from 60 rather than lining up the 100s and 10s.)
Do you agree with the solution? (Possible response: The answer is correct but the strategy is different than I have seen.)
Why doesn’t it matter? (Possible response: Because the value of each part is represented, the actual numbers can be subtracted anywhere.)
Solve 152 − 83. (Possible response: 100 − 80 = 20 and 50 − 3 = 47. 47 + 20 + 2 = 69.)