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Subtracting Decimals Demystified

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Lesson Plan

Subtracting Decimals Demystified

Students will be able to accurately subtract decimals by aligning the decimal points and subtracting digits in corresponding place values.

Mastering decimal subtraction is crucial for everyday financial literacy, understanding measurements, and succeeding in more advanced mathematical topics.

Audience

7th Grade Students

Time

30 minutes

Approach

Direct instruction, guided practice, and interactive fill-in-the-notes.

Materials

Subtracting Decimals Demystified Slide Deck, Subtracting Decimals Fill-in-Notes, Whiteboard or projector, and Markers or pens

Prep

Prepare Materials

5 minutes

Step 1

Warm-Up: Decimal Review

5 minutes

Step 2

Introduction to Subtracting Decimals

10 minutes

  • Display Slide 2 of the Subtracting Decimals Demystified Slide Deck and explain the key rule: ALWAYS align the decimal points.
    - Guide students through filling in the corresponding section on their Subtracting Decimals Fill-in-Notes.
    - Show Slide 3 to demonstrate how to add zeros as placeholders.
    - Work through the first example on Slide 4 together, modeling each step and having students fill in their notes.

Step 3

Guided Practice

10 minutes

  • Present Slide 5 with a new example.
    - Ask students to try the first few steps on their own using their Subtracting Decimals Fill-in-Notes and then review as a class.
    - Use Slide 6 and Slide 7 for additional practice problems, allowing students to work independently or in pairs before reviewing the answers.
    - Circulate around the room, providing support and checking for understanding.

Step 4

Quick Check & Wrap-Up

5 minutes

  • Project Slide 8 with a quick problem for students to solve independently.
    - Review the answer as a class.
    - Ask students to summarize the most important rule for subtracting decimals.
    - Collect the Subtracting Decimals Fill-in-Notes to check for completion and understanding.
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Slide Deck

Warm-Up: What's a Decimal?

What do you remember about decimals and place value? How are they used in real life?

Greet students and start with a quick warm-up question to activate prior knowledge about decimals.

The Golden Rule!

ALWAYS align the decimal points!

  • Think of it like stacking blocks perfectly.
  • This ensures you subtract digits with the same place value.

Introduce the main rule for subtracting decimals. Emphasize the importance of lining up the decimal points. Students should fill this into their notes.

Zeros as Placeholders

Sometimes, you'll need to add zeros to the end of a decimal to make the columns line up.

Example: 5.2 - 1.05

Think: 5.20 - 1.05

Explain how zeros act as placeholders and why they are important for accurate subtraction.

Example 1: Let's Do It Together!

Subtract: 7.5 - 3.25

  1. Align the decimal points.

  2. Add zeros if needed.

  3. Subtract from right to left, borrowing when necessary.

    7.50

  • 3.25
4.25

Go through the first example step-by-step. Model borrowing if necessary. Students should follow along in their notes.

Example 2: Your Turn to Start!

Subtract: 12.8 - 5.37

  1. Align the decimal points.
  2. Add zeros if needed.
  3. Subtract from right to left.

Try setting it up on your own first!

Present a new example and have students try the first few steps independently before reviewing as a class.

Practice Time!

Solve: 9.00 - 4.15

Remember the steps!




Another practice problem. Encourage students to work with a partner briefly.

Challenge Yourself!

Solve: 15.1 - 7.892

Don't forget the placeholders!




A slightly more challenging problem, maybe involving more borrowing or more digits. Review the solution.

Quick Check!

What is the most important rule to remember when subtracting decimals?




Solve: 10 - 6.75

Final quick check to gauge understanding. Collect notes afterwards.

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Worksheet

Subtracting Decimals Demystified: Fill-in-Notes

The Golden Rule of Decimal Subtraction

When subtracting decimals, the most important rule is to ALWAYS __________________ the decimal points.

This makes sure you subtract numbers with the same place value.

Why is this important?

Imagine you have $5.50 and you spend $2.25. If you didn't line up the decimals, you might subtract dollars from cents, which wouldn't make sense!

Zeros as Placeholders

Sometimes, the numbers won't have the same number of digits after the decimal point. When this happens, you can add ________ to the end of the decimal that has fewer digits. This doesn't change the value of the number, but it helps keep your columns organized.

Example: 5.2 - 1.05

We can rewrite this as: 5.2__ - 1.05

Steps to Subtract Decimals

  1. Align the __________________ points.
  2. Add __________________ as placeholders if needed.
  3. Subtract from right to left, just like with whole numbers. Remember to __________________ if necessary!

Example 1: Let's Do It Together!

Subtract: 7.5 - 3.25

  1. Align: 
    7.5

  • 3.25

  1. Add Zeros: 
    7.50

  • 3.25

  1. Subtract: 
    7.50

  • 3.25

Example 2: Your Turn to Start!

Subtract: 12.8 - 5.37

Set it up and solve!












Practice Time!

Solve: 9.00 - 4.15












Challenge Yourself!

Solve: 15.1 - 7.892












Quick Check!

What is the most important rule to remember when subtracting decimals?




Solve: 10 - 6.75












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