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2.7: Decimal Subtraction

Difficulty Level: At Grade Created by: CK-12
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Let’s Think About It

Owen likes to help his mom cut out coupons from the paper on Sunday mornings. This week he found 5 coupons that will save them $4.10 on their purchases at the grocery store. Once Owen and his mom get to the grocery store, they fill up their cart with everything on their list. At the cash register, their total is $85.91 before Owen hands over the coupons. How could Owen determine what the new total is going to be after the coupons are applied?

In this concept, you will learn how to subtract decimals with and without rounding.

Guidance

Recall that in a decimal number, the decimal point divides the whole part of the number from the fractional part of the number.

To subtract decimal numbers:

  1. Arrange the numbers vertically so that the decimal points line up.
  2. Add zeros for any missing spaces so that each number has the same number of digits to the right of the decimal point.
  3. Subtract from right to left, borrowing as necessary just as if you were subtracting whole numbers.
  4. Insert a decimal point into your answer so that it lines up with the decimal points of the original numbers.

Here is an example.

Subtract 4.5 from 18.98.

First, notice that you are going to find 18.98 minus 4.5.

Now, arrange the numbers vertically so that the decimal points line up.

\begin{align*}& \ \ 18.98\\ & \underline{- \ 4.5}\end{align*}  18.98 4.5

Next, add a zero after the 5 in the second number so that both numbers have two digits to the right of the decimal point.

\begin{align*}& \ \ 18.98\\ & \underline{- \ 4.50}\end{align*}  18.98 4.50

Now, subtract from right to left. Start by subtracting 8 minus 0 to make 8. Then 9 minus 5 to make 4. Then 18 minus 4 to make 14. Insert a decimal point into your answer directly under the decimal points in the original numbers.

\begin{align*}& \ \ 18.98\\ & \underline{- \ 4.50}\\ & \ \ 14.48\end{align*}  18.98 4.50  14.48

The answer is \begin{align*}18.98-4.5=14.48\end{align*}18.984.5=14.48.

Sometimes you will want to round numbers before subtracting them. Rounding is useful when you only need an approximate answer instead of an exact answer. Read the problem carefully to determine if you should round before or after you subtract.

Here is an example.

Round each number to the nearest tenth and then find the difference.

\begin{align*}72.953-52.418\end{align*}72.95352.418 

First, round each number to the nearest tenth. Remember that the tenths place is the first digit to the right of the decimal point. Look at each digit in the tenths place and then the digit to the right of that to decide if you will need to round up the digit in the tenths place or keep it the same.

  • 72.953 rounds to 73.0
  • 52.418 rounds to 52.4

Now, subtract the rounded numbers. Start by arranging the numbers vertically so that the decimal points line up.

\begin{align*}& \ \ \ \ 73.0\\ & \underline{- \ 52.4}\end{align*}    73.0 52.4

Next, subtract from right to left. Borrow from the 3 and then subtract 10 minus 4 to make 6. Then subtract 2 minus 2 to make 0. Finally subtract 7 minus 5 to make 2. Insert a decimal point into your answer directly under the decimal points in the original numbers.

\begin{align*}& \ \ \ \ 73.0\\ & \underline{- \ 52.4} \\ & \ \ \ \ 20.6\end{align*}    73.0 52.4    20.6

The answer is \begin{align*}73.0-52.4=20.6\end{align*}73.052.4=20.6.

Guided Practice

Subtract \begin{align*}4.56-2.37\end{align*}4.562.37.

First, arrange the numbers vertically so that the decimal points line up.

\begin{align*}& \ \ \ \ 4.56\\ & \underline{- \ 2.37} \end{align*}    4.56 2.37

Now, subtract from right to left. Start by borrowing from the 5 and then subtracting 16 minus 7 to make 9. Then subtract 4 minus 3 to make 1. Finally subtract 4 minus 2 to make 2. Insert a decimal point into your answer directly under the decimal points in the original numbers.

\begin{align*}& \ \ \ \ 4.56\\ & \underline{- \ 2.37} \\ & \ \ \ \ 2.19\end{align*}    4.56 2.37    2.19

The answer is \begin{align*}4.56-2.37=2.19\end{align*}4.562.37=2.19.

Examples

Example 1

Subtract \begin{align*}5.674-2.5\end{align*}5.6742.5.

First, arrange the numbers vertically so that the decimal points line up.

\begin{align*}& \ \ \ \ 5.674 \\ & \underline{- \ 2.5 \;\;\;\;\;}\end{align*}    5.674 2.5

Next, add two zeros after the 5 in the second number so that both numbers have three digits to the right of the decimal point.

\begin{align*}& \ \ \ \ 5.674 \\ & \underline{- \ 2.500}\end{align*}    5.674 2.500

Now, subtract from right to left. Start by subtracting 4 minus 0 to make 4. Then 7 minus 0 to make 7. Then 6 minus 5 to make 1. Finally 5 minus 2 to make 3. Insert a decimal point into your answer directly under the decimal points in the original numbers.

\begin{align*}& \ \ \ \ 5.674 \\ & \underline{- \ 2.500}\\ & \ \ \ \ 3.174\end{align*}    5.674 2.500    3.174

The answer is \begin{align*}5.674-2.5=3.174\end{align*}5.6742.5=3.174.

Example 2

Take 5.67 from 12.378.

First, notice that you are going to find 12.378 minus 5.67.

Now, arrange the numbers vertically so that the decimal points line up.

\begin{align*}& \ \ 12.378\\ & \underline{- \ 5.67 \;\;}\end{align*}  12.378 5.67

Next, add a zero after the 7 in the second number so that both numbers have three digits to the right of the decimal point.

\begin{align*}& \ \ 12.378\\ & \underline{- \ 5.670}\end{align*}  12.378 5.670

Now, subtract from right to left. Start by subtracting 8 minus 0 to make 8. Then 7 minus 7 to make 0. Then borrow from the 2 and subtract 13 minus 6 to make 7. Finally subtract 11 minus 5 to make 6. Insert a decimal point into your answer directly under the decimal points in the original numbers.

\begin{align*}& \ \ 12.378\\ & \underline{- \ 5.670}\\ & \ \ \ \ 6.708\end{align*}  12.378 5.670    6.708

The answer is \begin{align*}12.378-5.67=6.708\end{align*}12.3785.67=6.708.

Example 3

Round to the nearest tenth and then subtract: \begin{align*}83.56-1.258\end{align*}83.561.258.

First, round each number to the nearest tenth. Remember that the tenths place is the first digit to the right of the decimal point. Look at each digit in the tenths place and then the digit to the right of that to decide if you will need to round up the digit in the tenths place or keep it the same.

  • 83.56 rounds to 83.6
  • 1.258 rounds to 1.3

Now, subtract the rounded numbers. Start by arranging the numbers vertically so that the decimal points line up.

\begin{align*}& \ \ 83.6\\ & \underline{- \ 1.3}\end{align*}  83.6 1.3

Next, subtract from right to left. Subtract 6 minus 3 to make 3. Then 83 minus 1 to make 82. Insert a decimal point into your answer directly under the decimal points in the original numbers.

\begin{align*}& \ \ 83.6\\ & \underline{- \ 1.3} \\ & \ \ 82.3\end{align*}  83.6 1.3  82.3

The answer is \begin{align*}83.6-1.3=82.3\end{align*}83.61.3=82.3.

Follow Up

Remember Owen who likes clipping coupons with his mom? This week Owen found $4.10 worth of coupons. At the store, their items cost $85.91 before the coupons are applied. Owen wants to determine what their total will be after he gives the coupons to the cashier.

First, Owen needs to realize that he will need to subtract 85.91 minus 4.10.

Then, he should arrange the numbers vertically so that the decimal points line up.

\begin{align*}& \ \ 85.91\\ & \underline{- \ 4.10}\end{align*}  85.91 4.10

Now, he should subtract from right to left. First subtract 1 minus 0 to make 1. Then 9 minus 1 to make 8. Then 85 minus 4 to make 81. He needs to insert a decimal point into his answer directly under the decimal points in the original numbers.

\begin{align*}& \ \ 85.91\\ & \underline{- \ 4.10} \\ & \ \ 81.81\end{align*}  85.91 4.10  81.81

The answer is \begin{align*}85.91-4.10=81.81\end{align*}85.914.10=81.81. The total will be $81.81 after the coupons are applied.

Video Review

https://www.youtube.com/watch?v=t8r-S9Zr41c&feature=youtu.be

Explore More

Find each difference.

1. \begin{align*}6.57 - 5.75\end{align*}6.575.75

2. \begin{align*}.0826 - .044\end{align*}.0826.044

3. \begin{align*}19.315 - 6.8116\end{align*}19.3156.8116

4. \begin{align*}2056.04 - 2044.1\end{align*}2056.042044.1

5. \begin{align*}303.45 - 112.05\end{align*}303.45112.05

6. \begin{align*}16.576 - 8.43\end{align*}16.5768.43

7. \begin{align*}199.2 - 123.45\end{align*}199.2123.45

8. \begin{align*}1.0009 - .234\end{align*}1.0009.234

9. \begin{align*}789.12 - .876\end{align*}789.12.876

10. \begin{align*}102.03 - .27\end{align*}102.03.27

Find the difference after rounding each decimal to the nearest hundredth.

11. \begin{align*}63.385 - 50.508\end{align*}63.38550.508

12. \begin{align*}.535 - .361\end{align*}.535.361

13. \begin{align*}747.005 - 47.035\end{align*}747.00547.035

14. \begin{align*}.882 - .546\end{align*}.882.546

15. \begin{align*}.9887 - .0245\end{align*}.9887.0245

Vocabulary

Decimal

Decimal

In common use, a decimal refers to part of a whole number. The numbers to the left of a decimal point represent whole numbers, and each number to the right of a decimal point represents a fractional part of a power of one-tenth. For instance: The decimal value 1.24 indicates 1 whole unit, 2 tenths, and 4 hundredths (commonly described as 24 hundredths).
Decimal point

Decimal point

A decimal point is a period that separates the complete units from the fractional parts in a decimal number.
Difference

Difference

The result of a subtraction operation is called a difference.
Estimate

Estimate

To estimate is to find an approximate answer that is reasonable or makes sense given the problem.
Magnitude

Magnitude

The magnitude of a number is the size of a number without respect to its sign. The number -35.6 has a magnitude of 35.6.
Place Value

Place Value

The value of given a digit in a multi-digit number that is indicated by the place or position of the digit.
Rounding

Rounding

Rounding is reducing the number of non-zero digits in a number while keeping the overall value of the number similar.

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Difficulty Level:
At Grade
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Date Created:
Dec 02, 2015
Last Modified:
Mar 23, 2016
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