Lesson 5

Subtraction

Est. Class Sessions: 2–3

Developing the Lesson

Part 5. Showing Work with Expanded Form

Students used expanded form in Lesson 4 to solve addition problems. They can also use expanded form to subtract. Use Questions 8 and 9 in the Student Guide to analyze the expanded form strategy. Ask students to solve the problems in Question 10 using expanded form.

Use these discussion prompts:

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  • Describe how expanded form is used in the first problem. (You break the number into a sum of ones, tens, and hundreds; then subtract each part separately.)
  • How is this method like other methods we have used? (The little numbers show the trades. The same trades are made; for example, 1 ten is traded for 10 ones.)
  • How is it different? (It doesn't use place value—it writes out the full number. For example, when we use the compact method, we write 15 in the tens column to show a trade. But in expanded form, we write 150. It means the same since 15 tens equals 150, but it is a different way of showing it.)

Writing out a solution in expanded form may help some students solidify their understanding of the trading involved in subtraction. The purpose of trading is to express the minuend in a way that makes it possible to subtract. The trades give a new partition of the same number. But students sometimes miss the point that it must be the same number and think the trades changed the value of the number.

Using expanded form is not a particularly efficient way to solve subtraction problems. However, writing a solution in expanded form can help students better understand the trades involved in subtracting.

Student Guide: 141
Answers
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SG_Mini
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Student Activity Book:
Master: