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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