Tips for mental calculation
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* Say it in your mind

When you calculate in your mind, often you have to take several steps to a solution.

For example:

12 x 36 =

Step 1: 10 x 36 = 360

Step 2: 2 x 36 = 72

Step 3: 360 + 72 = 432

It can be useful to say the outcome of the steps in your mind as you calculate them. So when you calculate 12 x 36, you would say 360, plus 72, equals 432.

By mentally saying these steps, they'll be stored more firmly in your short term memory.
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* Addition and subtraction

With the method described below, it becomes relatively easy to calculate big numbers in your mind. With a little practice, you can mentally add and subtract numbers in the millions.

- Addition

Start at the left and calculate one digit at a time.

For example:

4629 + 3463 =

Step 1, the first digit form the left: 4 + 3 = 7. Say "seven."

Step 2, the second digit from the left: 6 + 4 = 10. Because 10 is not a single digit, you have to add the first digit to the one from step 1. 7 + 1 = 8. So now the first digit is 8, and the second digit is 0. Say "eight zero."

Step 3, the third digit from the left: 2 + 6 = 8. Say "eight zero eight."

Step 4, the fourth digit from the left: 9 + 3 = 12. Once again, you have to add 1 to the previous step's digit. Say "eight zero nine two."

And that's the solution: 4629 + 3463 = 8092

- Subtraction

The method for subtraction is much like the one for addition. You start from the left again. With the addition method, you calculate one digit at the time, and add 1 to the previous digit when the current digit becomes a number of 10 or greater. With subtraction, instead of having to add 1 to the previous digit, you sometimes have to subtract 1 from the previous digit, when the current digit becomes a number below zero. You will see this in the following example:

4629 – 3463 =

Step 1, the first digit from the left: 4 – 3 = 1. Say "one."

Step 2, the second digit from the left: 6 – 4 = 2. Say "one two."

Step 3, the third digit from the left: 2 – 6. You see right away this will become a negative number. So you'll have to subtract 1 from the previous digit. So far, in step 1 and 2, we've memorized "one two." Subtract 1 from the last digit. Say "one one." You can now add the 1 you've subtracted from the previous digit as a 10 to the current digit. So 2 – 6 become 12 – 6 = 6. Say "one one six."

Step 4, the fourth digit from the left: 9 – 3 = 6. Say "one one six six." This is the solution: 4629 – 3463 = 1166
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* Multiplication

If a number ends with one or more zeroes: first cut the zeroes, then paste them behind the outcome.

For example:

200 x 80000 =

Step 1: cut all six zeroes.

Step 2: 2 x 8 = 16

Step 3: paste all six zeroes behind the outcome: 16000000

200 x 80000 = 16000000
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* Round off

First add the required number to get a round number, multiply, then subtract the added number.

For example:

7 x 96 = (7 x 100) – (7 x 4) = 700 – 28 = 672

- Times five

To find a x 5, calculate a x 10 and divide by 2.

For example:

5 x 67 = 670 ÷ 2 = 335