How To Add and Subtract Two Numbers Mentally

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I can’t calculate that! I studied arts!

Or humanities, or literature, or… the excuse is always the same for not being able to perform such simple mental calculations like additions or subtractions.

Mental calculation is one of the strengths of the brain that you must train. By training mental calculation you will benefit of a better memory for numbers and you will increase you focus power. On the practical side, you will get comfortable with calculations are found on everyday life.

The Technique

We always have been taught to add and subtract from right to left. But it happens that it is far more effective doing it the other way around, from left to right.

How does it work?

Addition

  • Given two numbers to add, you should position the largest one at the left part of the addition. For instance, if you have to add the numbers 13341 plus 54821 , you should place in your mind, first the number 54821 and then the 13341.
  • In order to better implement in your mind the left-to-right addition method, you must add one digit at a time, aligning the same units, that is units, tens, hundreds,… For the addition of the two numbers before, you should add 5+1, then 4+3, then 8+3, etc.
  • Everytime you add two cyphers, say the current result numbers loud in your mind the result. For the numbers above:

5 + 1 = 6 (say six)

4 + 3 = 7 (say six seven)

  • When the addition of two cyphers results in a two cyphers number, then you must add one to the previous calculation and repeat mentally the two cyphers. For instance, continuing with the previous example:

8 + 3 = 11 (is a two cyphers number)

because 11 is a two digits cypher, you must add a unit to the previous addition, i.e.

7 + 1 = 8 (then say six eight one)

  • Continue using the same method with the rest of cyphers:

2 + 4 = 6 (say six eight one six)

1 + 1 = 2 (say six eight one six two)

Final result: 68162 (six eight one six two)

Subtraction

The subtraction technique is very similar to the addition.

  • Subtract one digit at a time, aligning the same units.
  • Say in your mind the resulting numbers. For example, if we subtract the two numbers above (54821 minus 13341):

5 – 1 = 4 (say four)

4 – 3 = 1 (say four one)

8 – 3 = 5 (say four one five)

  • When the subtraction of two cyphers results in a negative number, you must do two things. For instance, if the numbers to subtract are :

2 – 4 = -2

    • first, subtract 1 to the previous cypher subtraction:

5 – 1 = 4

    • second, add 10 to the smaller cypher of the negative subtraction, and perform the subtraction as normal

From 24 = -2

Add 10 to the 2

2 + 10 = 12

Then subtract to that number the 4

12 – 4 = 8 (say four one four eight)

  •  Keep doing the same procedure until all the cyphers have been calculated

1 – 1 = 0 (say four one four eight zero)

Final result: 41480 (say four one four eight zero)

 

Now that you know the techniques for addition and subtraction it is time to train them with the following workouts. Read the workouts and keep a log of your training on the Training Workbook.

 

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The Workout

Preparation: download a program for your mobile phone or computer that generates random numbers. You can find a list of programs at the end of this posts.

Do the following workouts every day from Monday to Friday for five minutes each day. It will only take you 10 minutes per day. Repeat the training for two weeks and the technique will be yours.

Workout 1 : ADDITION

  1. Start the random numbers generator and configure it to generate numbers between 0 and 99999
  2. Start a timer for 5 minutes
  3. Generate two numbers and write them down on a paper
  4. Then add them mentally using the technique explained above
  5. When added, go back to step number 3
  6. Repeat until the five minutes have passed
  7. Write down on your Training Workbook the number of additions done in the five minutes
  8. Next day, try to beat that number

 

Workout 2: SUBTRACTION

  1. Start the random numbers generator and configure it to generate numbers between 0 and 99999
  2. Start a timer for 5 minutes
  3. Generate two numbers and write them down on a paper
  4. Then subtract them mentally using the technique explained above
  5. When subtracted, go back to step number 3
  6. Repeat until the five minutes have passed
  7. Write down on your Training Workbook the number of subtraction done in the five minutes
  8. Next day, try to beat that number

 

Mastering addition and subtraction mentally will be the basis for more complex mental calculation abilities like multiplication and division, that are explained in this post.

 

Start now training these skills and increase your brain power

 

List of random generator programs

 

photo credit: Akash k via photopin cc
photo credit: nickicolleen via photopin cc

2 Responses to How To Add and Subtract Two Numbers Mentally

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