Lesson: Add and Subtract Decimals

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Lesson Objective

SWBAT add and subtract decimals by lining up the decimal.

Lesson Plan



Kurt Sarsfield

5th Grade Math Problem Solving

Date: March 4, 2010


5.12--Add and Subtract Decimals


SWBAT add and subtract decimals by lining up the decimal.



Introduction to Lesson/Hook: 

Example 1

Show the how and why of adding decimals and regrouping using a visual of a 100 grid.



Today we will work on adding and subtracting decimals. 


Scaffolded Questions

How do you say the first addend?  Eight tenths

How do you say the second addend? thirty five hundredths.

Using the 100 grid below, how would you shade the grid to represent 8 tenths?  You would shade 80 little squares or 8 bars.

Predict Misunderstanding

Incorrect Answer: Shade 8 little squares.

If you shaded 8 little squares, what fraction would you shade? 8 hundredths.

Incorrect Answer:  8 tenths

Let’s think about 8 tenths as a fraction.  How would we write that?  8 out of 10

Do we have 8 out of 10 squares shaded? 

How many do we have shaded? 8 out of 100. 

What do we need to do to shade 8 tenths?  Shade 80 little squares.


We are adding 0.35. 

How do I shade 0.35 in our hundred grid?  Thirty five hundred squares or 3 “bars” and 5 little squares.

To push for deeper understanding:

When student gives one answer:   If we only had a grid that was divided into tenths, how would we shade 35 hundredths? 

Now, let’s write the decimal I have shaded.

What is our whole number? 1

How many whole grids do I have shaded?

What fraction is left over?

How do I write the amount that is left over?


Now, we have looked at a visual of adding decimals.  Let’s look at how to add these numbers using arithmetic.

First, line up the decimals.

If we had 8 tenths and 35 hundredths, would we be able to add these fractions?

What could our common denominator be?  100

What do we need to do to turn 8 tenths into a fraction with a denominator of 100?

If we multiply by 10, we will get how many hundredths?

Now how can we write this in our arithmetic?



Example 2

Show how to estimate answer by rounding to the whole number.  Estimate and then solve a decimal addition problem.   Teacher bringing down the decimal point.  


In example 2 we will practice adding and subtracting decimals by lining up the decimal points.

Before we add the decimals in a, we need to estimate to the nearest whole number to make sure our answer will be close.

What would you round 7. 1 to?  

What would you round 3.75 to?

How would you know that 7.1 is closer to 7?

How did you know that 3.75 is closer to 4?

What is the sum of our rounded addends?

Now add the decimals to see if your answer is close to your estimate.



Example 3

Estimate.  Subtract and show what to do when you need to add additional zeros.


Round each of the numbers we will subtract.

What is your estimated difference?

When we find our actual answer, what do we need to do with the decimal points?

Look above the 5. 

When there is nothing in the space, what do we need to write there?

Can you subtract 5 from 0?

What do I need to do so that I have an amount I can subtract from 0?  We need to borrow from the 1. 

Teacher shows borrowing from the tenths. 

Now, we also need to borrow from the ones. 

Push for deeper understanding

When we borrow a one from 7, how many tenths do we have to put in the tenths column?





 Exit Ticket

Lesson Resources

5.12 Add and subtract decimals 5Math ProbSolv Sarsfield 2010.03.04 BL.doc  


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