6th Grade Math Textbook, Progress

Think Think

Think

Mental Math: Addition and Subtraction

7-7

3 
 4

1

 1 
 2

1

n

1 
 21 
 21  2

3 
 41  2  1 
 41 and 1
 41 
 21  1 
 43

So 3
 41  1 
 21  1 
 43.

7  4 
 1

7

6


n

4 
 17 
 6 
 19 
 6  5

7  5 2 and 2 
 19 
 6  2 
 19 
 6

So 7 4 
 17 
 6  2 
 19 
 6.

You can add and subtract fractions and mixed numbers mentally

by using the three strategies below.

Compute the whole-number

part and then the fraction part.

Look for sums of 1.

Compensate by “adding on”

and then subtracting.

Study these examples.

Think

Think

The denominators are alike.

(85) (
 87 

 85 
)

 61 
 65 
 1

3 
 8

5


 8

3

 4

Subtract 4, then add 
 8

3

.

10  4 6 and 6
 8

3

 6 
 8

3

5 
 61  2 
 65 

^1 
 3  8 


 31

8 
^78  5 
 85  3 

 82  3 
 41 

10  3 
 85  6 

 83

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