Title: LESSON: Adding and Subtracting Decimals
Course: Mathematics Essentials Q1 > Section: Week 4

Adding and Subtracting Decimals

INTRODUCTION

Adding and Subtracting Decimals

READ: Adding and Subtracting Decimals by Rewriting

Adding and Subtracting Decimals by Rewriting With Additional Zero Place Holders

To add or subtract decimals, we are going to be working with the wholes and parts of the numbers separately. We want to add or subtract the parts and then add or subtract the wholes. How can we do this? The best way to do this is to keep the parts together and keep the wholes together. To do this, we simply line up the decimal points in each number that we are adding or subtracting.

Example

Add 3.45 + 2.37 = _____

In this problem we have parts and wholes. Let’s rewrite the problem vertically lining up the decimal points.

Next, we can add the columns vertically and bring the decimal point down into the answer of the problem.

Our answer is 5.82.

Does this work the same way when finding a difference?

Yes. We can line up the decimals in a subtraction problem and subtract the digits the same way.

Example

6.78 - 2.31 = _____

First, we line up the problem vertically.

Next, we subtract each digit vertically.

Our answer is 4.47.

Now these examples both had the same number of digits in them. They each had one whole number and a decimal in the hundredths. What happens when you have decimals with different numbers of digits in them? When we have a problem like this, we still line up the decimal points, but we add zeros to help hold places where there aren’t numbers. This helps us to keep our addition and subtraction straight.

Example

5 + 3.45 + .56 = _____

First, we line up the problem vertically.

Notice that we added in zeros to help hold places where we did not have numbers. Now each number in the problem has the same number of digits. We can add them with ease.

Our answer is 9.01.

We can do the same thing with a subtraction problem. We add zeros to help hold places where there are not digits. That way each number has the same number of places.

Example

67.89 - 18.4 = _____

First, we line up the problem vertically with the decimal point.

Our answer is 49.49.

WATCH: Modeling Addition and Subtraction of Decimals

WATCH: Addition of Decimals

EXAMPLE 1

CHECK Yourself! Adding Decimals

WATCH: Modeling Subtracting Decimals

WATCH: Subtraction of Decimals

EXAMPLE 2

CHECK Yourself! Subtract Decimals