Find the Answer Mentally by Addition & by Subtraction

H.D. Motiramani

Numbers that end with 5,6,7,8,9 can be made tens by addition of another small number. For example 9 can be considered or made to become ten if 1 is added to it. Likewise, 8 can become ten if 2 is added to 8. We use this simple technique to solve our problems of adding two numbers mentally.

Let’s see a problem:     48 + 9

                       By sub  -1   +1  By addition

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It will be simplified as  47 + 10 = 57

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Examples:

 

1.       27 + 69

-1     +1

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-26 + 70 = 96

 

2.       37  + 38

-2      +2

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    35  + 40 = 75

 

3.       24  +  29

-1        +1

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23 + 30 = 53

 

4.       19   +   18

+1        -1

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20   +   17 = 37

 

5.       78   +   59

-1         +1

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77   +   60 = 137

 

This method can very well be applied to higher digit numbers too.

 

Example:

 

278   +   459

-1         +1

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277   +   460

For such additions first add 77 to 60 to get 137. Then add 200+400= 600. Now add 600 to 137 to get 737 as answer.

 

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