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4.7: Add and Subtract Mixed Numbers

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Learning Objectives

By the end of this section, you will be able to:

  • Model addition of mixed numbers with a common denominator
  • Add mixed numbers with a common denominator
  • Model subtraction of mixed numbers
  • Subtract mixed numbers with a common denominator
  • Add and subtract mixed numbers with different denominators

Be Prepared 4.14

Before you get started, take this readiness quiz.

Draw figure to model 73.
If you missed this problem, review Example 4.6.

Be Prepared 4.15

Change 114 to a mixed number.
If you missed this problem, review Example 4.9.

Be Prepared 4.16

Change 312 to an improper fraction.
If you missed this problem, review Example 4.11.

Model Addition of Mixed Numbers with a Common Denominator

So far, we’ve added and subtracted proper and improper fractions, but not mixed numbers. Let’s begin by thinking about addition of mixed numbers using money.

If Ron has 1 dollar and 1 quarter, he has 114 dollars.

If Don has 2 dollars and 1 quarter, he has 214 dollars.

What if Ron and Don put their money together? They would have 3 dollars and 2 quarters. They add the dollars and add the quarters. This makes 324 dollars. Because two quarters is half a dollar, they would have 3 and a half dollars, or 312 dollars.

114+214________324=312

When you added the dollars and then added the quarters, you were adding the whole numbers and then adding the fractions.

114+214

We can use fraction circles to model this same example:

114+214
Start with 114. one whole and one 14 pieces . .
Add 214 more. two wholes and one 14 pieces . .
The sum is: three wholes and two 14's . .

Manipulative Mathematics

Example 4.81

Model 213+123 and give the sum.

Answer

We will use fraction circles, whole circles for the whole numbers and 13 pieces for the fractions.

two wholes and one 13 . .
plus one whole and two 13s . .
sum is three wholes and three 13s . .

This is the same as 4 wholes. So, 213+123=4.

Try It 4.161

Use a model to add the following. Draw a picture to illustrate your model.

125+335

Try It 4.162

Use a model to add the following. Draw a picture to illustrate your model.

216+256

Example 4.82

Model 135+235 and give the sum as a mixed number.

Answer

We will use fraction circles, whole circles for the whole numbers and 15 pieces for the fractions.

one whole and three 15s . .
plus two wholes and three 15s. . .
sum is three wholes and six 15s . .

Adding the whole circles and fifth pieces, we got a sum of 365. We can see that 65 is equivalent to 115, so we add that to the 3 to get 415.

Try It 4.163

Model, and give the sum as a mixed number. Draw a picture to illustrate your model.

256+156

Try It 4.164

Model, and give the sum as a mixed number. Draw a picture to illustrate your model.

158+178

Add Mixed Numbers

Modeling with fraction circles helps illustrate the process for adding mixed numbers: We add the whole numbers and add the fractions, and then we simplify the result, if possible.

How To

Add mixed numbers with a common denominator.

Step 1. Add the whole numbers.

Step 2. Add the fractions.

Step 3. Simplify, if possible.

Example 4.83

Add: 349+229.

Answer

349+229
Add the whole numbers. .
Add the fractions. .
Simplify the fraction. .

Try It 4.165

Find the sum: 447+127.

Try It 4.166

Find the sum: 2311+5611.

In Example 4.83, the sum of the fractions was a proper fraction. Now we will work through an example where the sum is an improper fraction.

Example 4.84

Find the sum: 959+579.

Answer

959+579
Add the whole numbers and then add the fractions.
959+579_____14129
Rewrite 129 as an improper fraction. 14+139
Add. 1539
Simplify. 1513

Try It 4.167

Find the sum: 878+758.

Try It 4.168

Find the sum: 679+859.

An alternate method for adding mixed numbers is to convert the mixed numbers to improper fractions and then add the improper fractions. This method is usually written horizontally.

Example 4.85

Add by converting the mixed numbers to improper fractions: 378+438.

Answer

378+438
Convert to improper fractions. 318+358
Add the fractions. 31+358
Simplify the numerator. 668
Rewrite as a mixed number. 828
Simplify the fraction. 814

Since the problem was given in mixed number form, we will write the sum as a mixed number.

Try It 4.169

Find the sum by converting the mixed numbers to improper fractions:

559+379.

Try It 4.170

Find the sum by converting the mixed numbers to improper fractions:

3710+2910.

Table 4.2 compares the two methods of addition, using the expression 325+645 as an example. Which way do you prefer?

Mixed Numbers Improper Fractions
325+6459659+659+1151015 325+645175+3455151015
Table 4.2

Model Subtraction of Mixed Numbers

Let’s think of pizzas again to model subtraction of mixed numbers with a common denominator. Suppose you just baked a whole pizza and want to give your brother half of the pizza. What do you have to do to the pizza to give him half? You have to cut it into at least two pieces. Then you can give him half.

We will use fraction circles (pizzas!) to help us visualize the process.

Start with one whole.

A shaded circle is shown. Below it is a 1. There are arrows pointing to a shaded circle divided into 2 equal parts. Below it is 2 over 2. Next to this are two circles, each divided into 2 equal parts. The top circle has the right half shaded and the bottom circle has the left half shaded.

Algebraically, you would write:

On the left, it says 1 minus 1 half. There is an arrow pointing to 2 over 2 minus 1 over 2. There is another arrow pointing to 2 over 2 minus 1 over 2 equals 1 over 2.

Example 4.86

Use a model to subtract: 113.

Answer

There is a table with five rows and three columns. The first column is not labeled. The second column is labeled “Model.” The third column is labeled “Math Notation.” In the first column, the first row says, “Rewrite vertically. Start with one whole.” The next row says, “Since one-third has denominator 3, cut the whole into 3 pieces. The 1 whole becomes 3 thirds.” The next row says, “Take away one-third.” The last row says, “There are two-thirds left.” In the “Model” column, there is a picture of a shaded circle. Below that is a picture of a shaded circle divided into 3 equal pieces. Below that is a picture of a circle divided into 3 equal pieces with 2 pieces shaded. In the “Math Notation” column, the first row shows 1 minus 1 third. The next row says 3 thirds minus 1 third. The last row says 3 thirds minus 1 third is 2 thirds.

Try It 4.171

Use a model to subtract: 114.

Try It 4.172

Use a model to subtract: 115.

What if we start with more than one whole? Let’s find out.

Example 4.87

Use a model to subtract: 234.

Answer

There is a table with four rows and three columns. The first column is not labeled. The second column is labeled “Model.” The third column is labeled “Math Notation.” In the first column, the first row says, “Rewrite vertically. Start with two wholes.” The next row says, “Since three-fourths has denominator 4, cut one of the wholes into 4 pieces. You have one whole and 4 fourths.” The next row says, “Take away three-fourths.” The last row says, “There is 1 and 1 fourth left.” In the “Model” column, there is a picture of two shaded circles. Below that is a picture of two shaded circles. One of the circles is divided into 4 equal pieces. Below that is a picture of one full shaded circle and a circle divided into 4 equal pieces with 1 piece shaded. In the “Math Notation” column, the first row shows 2 minus 3 fourths. The next row says 1 and 4 fourths minus 3 fourths. The last row says 1 and 4 fourths minus 3 fourths equals 1 and 1 fourth.

Try It 4.173

Use a model to subtract: 215.

Try It 4.174

Use a model to subtract: 213.

In the next example, we’ll subtract more than one whole.

Example 4.88

Use a model to subtract: 2125.

Answer

There is a table with five rows and three columns. The first column is not labeled. The second column is labeled “Model.” The third column is labeled “Math Notation.” In the first column, the first row says, “Rewrite vertically. Start with two wholes.” The next row says, “Since two-fifths has denominator 5, cut one of the wholes into 5 pieces. You have one whole and 5 fifths.” The next row says, “Take away 1 and two-fifths.” The last row says, “There is 3 fifths left.” In the “Model” column, there is a picture of two shaded circles. Below that is a picture of two shaded circles. One of the circles is divided into 5 equal pieces. Below that is a picture of one full unshaded circle and a circle divided into 5 equal pieces with 3 pieces shaded. In the “Math Notation” column, the first row shows 2 minus 1 and 2 fifths. The next row says 1 and 5 fifths minus 1 and 2 fifths. The last row says 1 and 5 fifths minus 1 and 2 fifths equals 3 fifths.

Try It 4.175

Use a model to subtract: 2113.

Try It 4.176

Use a model to subtract: 2114.

What if you start with a mixed number and need to subtract a fraction? Think about this situation: You need to put three quarters in a parking meter, but you have only a $1 bill and one quarter. What could you do? You could change the dollar bill into 4 quarters. The value of 4 quarters is the same as one dollar bill, but the 4 quarters are more useful for the parking meter. Now, instead of having a $1 bill and one quarter, you have 5 quarters and can put 3 quarters in the meter.

This models what happens when we subtract a fraction from a mixed number. We subtracted three quarters from one dollar and one quarter.

We can also model this using fraction circles, much like we did for addition of mixed numbers.

Example 4.89

Use a model to subtract: 11434

Answer

Rewrite vertically. Start with one whole and one fourth. . .
Since the fractions have denominator 4, cut the whole into 4 pieces.
You now have 44 and 14 which is 54.
. .
Take away 34.
There is 12 left.
. .

Try It 4.177

Use a model to subtract. Draw a picture to illustrate your model.

11323

Try It 4.178

Use a model to subtract. Draw a picture to illustrate your model.

11545

Subtract Mixed Numbers with a Common Denominator

Now we will subtract mixed numbers without using a model. But it may help to picture the model in your mind as you read the steps.

How To

Subtract mixed numbers with common denominators.
  1. Step 1. Rewrite the problem in vertical form.
  2. Step 2. Compare the two fractions.
    • If the top fraction is larger than the bottom fraction, go to Step 3.
    • If not, in the top mixed number, take one whole and add it to the fraction part, making a mixed number with an improper fraction.
  3. Step 3. Subtract the fractions.
  4. Step 4. Subtract the whole numbers.
  5. Step 5. Simplify, if possible.

Example 4.90

Find the difference: 535245.

Answer

.
Rewrite the problem in vertical form. .
Since 35 is less than 45, take 1 from the 5 and add it to the 35:(55+35=85) .
Subtract the fractions. .
Subtract the whole parts.
The result is in simplest form.
.

Since the problem was given with mixed numbers, we leave the result as mixed numbers.

Try It 4.179

Find the difference: 649379.

Try It 4.180

Find the difference: 447267.

Just as we did with addition, we could subtract mixed numbers by converting them first to improper fractions. We should write the answer in the form it was given, so if we are given mixed numbers to subtract we will write the answer as a mixed number.

How To

Subtract mixed numbers with common denominators as improper fractions.

Step 1. Rewrite the mixed numbers as improper fractions.

Step 2. Subtract the numerators.

Step 3. Write the answer as a mixed number, simplifying the fraction part, if possible.

Example 4.91

Find the difference by converting to improper fractions:

961171011.

Answer

961171011
Rewrite as improper fractions. 105118711
Subtract the numerators. 1811
Rewrite as a mixed number. 1711

Try It 4.181

Find the difference by converting the mixed numbers to improper fractions:

649379.

Try It 4.182

Find the difference by converting the mixed numbers to improper fractions:

447267.

Add and Subtract Mixed Numbers with Different Denominators

To add or subtract mixed numbers with different denominators, we first convert the fractions to equivalent fractions with the LCD. Then we can follow all the steps we used above for adding or subtracting fractions with like denominators.

Example 4.92

Add: 212+523.

Answer

Since the denominators are different, we rewrite the fractions as equivalent fractions with the LCD, 6. Then we will add and simplify.

There are three vertical addition problems. The first shows 2 and 1 half plus 5 and 2 thirds. There is an arrow pointing to the next. This one shows 2 and 1 times a red 3 over 2 times a red 3, with an arrow pointing to the top red 3 that says, “Change into equivalent,” plus 5 and 2 times a red 2 over 3 times a red 2. There is an arrow pointing to the next. This one shows 2 and 3 sixths plus 5 and 4 sixths equals 7 and 7 sixths. Below are instructions to add and rewrite in simplest form. There is an arrow pointing to a red 8 and 1 sixth.

We write the answer as a mixed number because we were given mixed numbers in the problem.

Try It 4.183

Add: 156+434.

Try It 4.184

Add: 345+812.

Example 4.93

Subtract: 434278.

Answer

Since the denominators of the fractions are different, we will rewrite them as equivalent fractions with the LCD 8. Once in that form, we will subtract. But we will need to borrow 1 first.

There are four vertical subtraction problems. The first shows 4 and 3 fourths minus 2 and 7 eighths. There is an arrow pointing to the next. This shows 4 and 3 times a red 2 over 4 times a red 2, with an arrow above saying, “change into equivalent,” minus 2 and 7 eighths. There is an arrow pointing to the next. This shows 4 and 6 eighths minus 2 and 7 eighths. There is an arrow pointing to the next. It says to borrow 1 whole from the 4, since we cannot subtract 7 eighths from 6 eighths, and shows 3 and 14 eighths minus 2 and 7 eighths equals 1 and 7 eighths.

We were given mixed numbers, so we leave the answer as a mixed number.

Try It 4.185

Find the difference: 812345.

Try It 4.186

Find the difference: 434156.

Example 4.94

Subtract: 3511434.

Answer

We can see the answer will be negative since we are subtracting 4 from 3. Generally, when we know the answer will be negative it is easier to subtract with improper fractions rather than mixed numbers.

3511434
Change to equivalent fractions with the LCD. 35·411·443·114·11

3204443344
Rewrite as improper fractions. 1524420944
Subtract. 5744
Rewrite as a mixed number. −11344

Try It 4.187

Subtract: 134678.

Try It 4.188

Subtract: 10372249.

Media

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Section 4.6 Exercises

Practice Makes Perfect

Model Addition of Mixed Numbers

In the following exercises, use a model to find the sum. Draw a picture to illustrate your model.

436.

115+315

437.

213+113

438.

138+178

439.

156+156

Add Mixed Numbers with a Common Denominator

In the following exercises, add.

440.

513+613

441.

249+519

442.

458+938

443.

7910+3110

444.

345+645

445.

923+123

446.

6910+8310

447.

849+289

Model Subtraction of Mixed Numbers

In the following exercises, use a model to find the difference. Draw a picture to illustrate your model.

448.

11656

449.

11858

Subtract Mixed Numbers with a Common Denominator

In the following exercises, find the difference.

450.

278138

451.

27121512

452.

817204920

453.

19131513715

454.

837447

455.

529349

456.

258178

457.

25121712

Add and Subtract Mixed Numbers with Different Denominators

In the following exercises, write the sum or difference as a mixed number in simplified form.

458.

314+613

459.

216+534

460.

158+412

461.

723+812

462.

9710213

463.

645114

464.

223312

465.

278413

Mixed Practice

In the following exercises, perform the indicated operation and write the result as a mixed number in simplified form.

466.

258·134

467.

123·416

468.

27+47

469.

29+59

470.

1512÷112

471.

2310÷110

472.

135129712

473.

1558678

474.

5949

475.

1115715

476.

434

477.

625

478.

920÷34

479.

724÷143

480.

9611+71011

481.

8513+4913

482.

325+534

483.

256+415

484.

815·1019

485.

512·89

486.

678213

487.

659425

488.

529445

489.

438323

Everyday Math

490.

Sewing Renata is sewing matching shirts for her husband and son. According to the patterns she will use, she needs 238 yards of fabric for her husband’s shirt and 118 yards of fabric for her son’s shirt. How much fabric does she need to make both shirts?

491.

Sewing Pauline has 314 yards of fabric to make a jacket. The jacket uses 223 yards. How much fabric will she have left after making the jacket?

492.

Printing Nishant is printing invitations on his computer. The paper is 812 inches wide, and he sets the print area to have a 112-inch border on each side. How wide is the print area on the sheet of paper?

493.

Framing a picture Tessa bought a picture frame for her son’s graduation picture. The picture is 8 inches wide. The picture frame is 258 inches wide on each side. How wide will the framed picture be?

Writing Exercises

494.

Draw a diagram and use it to explain how to add 158+278.

495.

Edgar will have to pay $3.75 in tolls to drive to the city.

Explain how he can make change from a $10 bill before he leaves so that he has the exact amount he needs.

How is Edgar’s situation similar to how you subtract 10334?

496.

Add 4512+378 twice, first by leaving them as mixed numbers and then by rewriting as improper fractions. Which method do you prefer, and why?

497.

Subtract 3784512 twice, first by leaving them as mixed numbers and then by rewriting as improper fractions. Which method do you prefer, and why?

Self Check

After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

.

After reviewing this checklist, what will you do to become confident for all objectives?


This page titled 4.7: Add and Subtract Mixed Numbers is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by OpenStax.

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