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Practice Completing Addition and Subtraction Sentences with Decimals

Instructor: Bethany Calderwood

Bethany has taught special education in grades PK-5 and has a master's degree in special education.

Decimal numbers occur in many different contexts. It is important to know how to use them in basic operations, such as addition and subtraction. In this lesson, you will practice adding and subtracting decimal numbers.

Road Trip Math

Rodrigo is taking a road trip with his family. He wants to keep track of how many miles they travel. Their odometer reports mileage using decimal numbers. Decimals show numbers that are parts of a whole. The numbers after the decimal point represent tenths, hundredths, and even smaller parts. To do his calculations, Rodrigo will need to add and subtract decimals.

Day One: Adding and Subtracting Decimals

On the first day, the family goes 23.12 miles before stopping at a museum. Then, they go 14.26 miles to a pizza place for lunch. Rodrigo wants to add those distances together.

23.12 + 14.26 = ?

After lunch, they go 44.5 miles farther. They missed the exit for their hotel, so they drive 3.3 miles back the way they came. To calculate their after-lunch travel, Rodrigo will need to subtract.

44.5 - 3.3 = ?

Both of these problems can be solved in a similar fashion. Look at the equations:


addition problem


The most important rule in operations with decimals is to line up the decimal points. The numbers to the left of the decimal point represent whole numbers, and the numbers to the right represent parts of a whole, or numbers less than one.

Once the decimals are lined up, complete the addition and subtraction. Here are the results:

23.12 + 14.26 = 37.38

And:

44.5 - 3.3 = 41.2

Day Two: Adding Zeroes

On the second day of the road trip, Rodrigo's family begins by traveling 17.1 miles to a park, where they hike all day. After their hike, they drive 29.34 miles to a new hotel. Rodrigo needs to add these numbers to find out their day two mileage.

17.1 + 29.34 = ?

When Rodrigo tries to line up the decimals, he sees a problem. The number 17.1 has only one number after the decimal point, while 29.34 has two numbers after the decimal point. He decides to add a zero to make 17.1 into 17.10.

It's okay for Rodrigo to add a zero after the 17.1 because the zero does not change the place value of the numbers already there. The number 17.1 already has a value of 0 in the hundredths place, Rodrigo is just writing the 0 there as a place holder so that he can do the math more easily.


addition problem


Rodrigo finds that 17.1 + 29.34 = 46.44

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