ADDING AND SUBTRACTING

DECIMALSLesson 1.01

Lesson Objectives

After completing this lesson, you will be able to say:

I can add decimals

I can subtract decimals

Place Value

In order to be able to add or subtract decimals you must be able to read the numbers according to place value.

Place Value

Let’s look at the place value of the digits in the number 65.483

If you line up the decimal with the word “and” then fill in your numbers you can quickly see the place value for each digit.

What does ESTIMATIONmean?

What does ESTIMATIONmean?

To find an approximate amount

The symbol "≈" means "is approximately equal to."

Estimation

• We can use estimation we can find an approximate amount.

One method of estimation is to round a number to a specific place value. You round a number by determining if the digit to the right of the specified place value is greater than or equal to 5. If it is greater than or equal to 5, you round the number up one digit. If the digit is less than 5, it remains the same

Example:

Using Estimation to Add or SubtractHow can we use estimation in this problem?Mary went to the video game store and bought three used games, which cost $9.57, $13.28, and $15.82. What was the total approximate cost of all three games if you round to the nearest dollar?

1. Round each number the nearest whole number $9.57 ≈ $10$13.28 ≈ $13$15.82 ≈ $16

2. Find the sum To find the sum, add up all the prices. The total is the sum.10 + 13 + 16 ≈ 39

Try It!

• An online map shows that Joey lives 87.3 miles away from his favorite mall. If he has already driven 32.5 miles, about how many whole miles farther does he still need to drive?

Check it!

An online map shows that Joey lives 87.3 miles away from his favorite mall. If he has already driven 32.5 miles, about how many whole miles farther does he still need to drive?

What do you think?

 Which do you think is the correct way to find the sum of 47.62 and 12.3?

What do you think?

 Which do you think is the correct way to find the sum of 47.62 and 12.3?

As you can see, place value is very important when adding or subtracting decimals.

Adding Decimals

Adding Decimals

Determine the sum for the following: 5.439 + 3.2 =

1. 5.439 Line up the decimals by their place value.+ 3.2 If you can not drop a straight line through your decimal points you have not lined them up

correctly.

1. 5.439 Add zeros as place value holders+ 3.200

Add each column vertically starting with the smallest place value position 5.439+ 3.200 8 639Bring your decimal point straight down. 5.439+ 3.200 8.639

Try it!

Try it!

Subtracting Decimals

Base on what you already know, which one do you think is correct?

Subtracting Decimals

Base on what you already know, which one do you think is correct?

Subtracting Decimals

Subtracting Decimals

What is the difference: 74.8 − 35.12 =

Adding Decimals

What is the difference: 74.8 − 35.12 =

1. 74.8 Line up the decimals by their place value.- 35.12 If you can not drop a straight line through your decimal points you have not lined them up

correctly.

74.80 Add zeros as place value holders- 35.12

Subtract each column vertically starting with the smallest place value position 74.80- 35.12 74.80- 35.12 Bring your decimal straight down. 39.68

Caution – make sure if subtraction is not possible you use your regrouping skills to subtract

Try It!

Try It!

Caution!!!

Caution!!!

Sum it up

When adding or subtracting, it is always helpful to follow these standard steps:

Step 1: Line up the decimals according to place value.

Step 2: Place 0s in the empty spaces after the decimal. 

Step 3: Add or subtract vertically and bring down the decimal point.

Now that you completed this lesson, you should be able to say:

I can add decimals!

I can subtract decimals!