Adding & Subtracting Decimals Using Models & Pictures

Instructor: Cassandra Cook

Cassandra Cook has taught grades K-8 and has a Specialist degree in Curriculum and Instruction

This lesson will teach you to add and subtract numbers with decimals using models. You will learn the process for adding and subtracting decimals, as well as how to accurately record the sums and differences.

What is a decimal?

A decimal is a point, or dot, that separates a whole number and a number less than one. A decimal number includes a whole number to the left of the decimal and the number whose value is less than one to the right of the decimal. Here are some examples:

Decimal Number Examples
Decimal Example

The small red dot is the decimal. When we include numbers to the left and right of the decimal, we have decimal numbers. You commonly see decimal numbers when dealing with money, such as $2.35 or $400.00 or even $35.87. When we put an amount of money in writing, we use a a decimal to represent the amount.

Adding Decimals

You are a cashier at the movie theater and today the registers have to be updated during your shift. This means you cannot use them! In order to make transactions, you have to add the popcorn, drinks, and food using pen and paper for each customer! Don't worry, it'll be easy! The first customer orders a large popcorn for $6.75, a medium soda for $5.25, and a bag of candy for $2.50. The first step to adding decimals is to line up all the decimals.

Decimals Lined Up
Lined Up Decimal

It does not matter the order of the numbers, as long as the decimal is lined up for each number. Next you want to bring your decimal straight down. This way you can be sure it is in the correct place in the answer. Since we are using money, we have to bring down the dollar sign as well.

Bring down the decimal and the dollar sign
Bring down decimal

Now you can add the numbers just like a regular addition problem.

Adding the decimal numbers
Adding the decimal numbers

As you see, the decimal is correctly placed to show that this customer's total is $14.50 for the purchase. This customer pays with exact change. The customer will not need any change back.

Your second customer orders two large bags of popcorn that cost $6.75 each, one large soda that costs $6.00, two bags of candy that costs $2.50 each, and one bottle of water that costs $2.25. How much will this customer's bill be? Let's add to find out. Remember to line your decimals up first, and then bring down the decimal in your answer.

$6.75 + $6.75 + $6.00 + $2.50 + $2.50 + $2.25 = $26.75

This customer's total is $26.75 for the snacks. The customer gives you $30.00 to pay, a $20.00 bill and a $10.00 bill. Now you have to give the customer change!

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