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3rd-5th Grade Math: Practice & Review37 chapters | 252 lessons

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Lesson Transcript

Instructor:
*Christine Quist*

Christine has taught 4th-5th grade, has worked as a Paraprofessional for Adult Learners, and has a master's degree in Curriculum and Instruction for Elementary Education.

In this lesson, you'll learn about that tiny dot we call a decimal point, which is used to show part of a whole number. While having fun with some real-life problems, you'll learn how to add, subtract, multiply, and divide with the decimal point.

The main place you will see a decimal point is when you are working with money. The **decimal point** is a divider between whole numbers and parts of a number. When you have two dollars and seventy-five cents ($2.75), you have two whole dollars and a part of the third dollar. The point between the dollars and the cents is the decimal point. Remember to line up your numbers by place value from left to right, as shown here:

Now, let's talk about adding decimals. Imagine you're at the market buying some carrots for soup. In one pocket you have thirty-five cents, and in the other pocket you have seventy-five cents. You need one whole dollar ($1.00) to buy the carrots. Now, let's find out if you have enough money:

1. First, line up the decimal points for the money: (.35 and .75)

2. Next, add the fives in the hundredths place to make one tenth, then carry the 1 so it's above the three in the tenths place

3. Add the 3 and the 7 in the tenths place to get 10 plus the 1 you carried, which will give you 1 whole number and 1 tenths: .35 + .75 = 1.10

At the end of the problem, make sure the decimal point always lands between the ones and tenths place. You have enough money!

Now, let's talk about subtracting with decimals. Imagine you're the store cashier. The carrots still cost $1.00, and the customer has given you too much money: $1.10. You'll have to make change:

1. To figure this out, first line up the decimal points, and then subtract as you would with whole numbers

2. After subtracting 0 from 0, place the result (which is 0) in the hundredths column

3. Subtract 0 from 1, and place the 1 in the tenths column

4. Now subtract the whole numbers to get 0

The change you owe the customer would be $.010 (ten cents).

When multiplying with decimals, there is a special trick to placing the point! Let's say you have to travel 3.5 miles to and from school every day five times a week. How many miles would you travel in one week?

1. Go ahead and set up your problem like this:

2. Thyen, multiply 5 by 5 to get 25, and carry the 2

3. Then multiply 5 times 3 to get 15; add the 2 you carried to get 17.

4. Here's the special trick: To place the decimal point, count the places to the right of the decimal point in each of the numbers in the original problem. In this case, there's only one number place behind the decimal point in one of the numbers in the original problem. So in your answer, the decimal point goes right before the 5, which means you travel 17.5 miles in a week.

Solving a division problem with decimal points is the same as solving a problem with no decimal points involved. When you've finished the problem, insert the decimal point in the same place above the bar as it is below the bar.

The **decimal point** is found mostly in problems involving money and is used to show a part of a whole. When using decimal points in addition and subtraction, remember to line up the numbers in their proper place value and then drop down the decimal point. In multiplication, count the numbers behind the decimal point in all the numbers in the original problem, and then put the total number of places behind the decimal point in the solution. When dividing, the decimal point occupies the same position above the bar as below the bar to complete the problem.

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3rd-5th Grade Math: Practice & Review37 chapters | 252 lessons

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