## Monday, October 11, 2010

### Fantastic Math Trick!

Multiply Up to 20X20 In Your Head

In just FIVE minutes you should learn to quickly multiply up to 20x20 in your head. With this trick, you will be able to multiply any two numbers from 11 to 19 in your head quickly, without the use of a calculator.

I will assume that you know your multiplication table reasonably well up to 10x10.

Try this:

Take 15 x 13 for an example.

Always place the larger number of the two on top in your mind.

Then draw the shape of Africa mentally so it covers the 15 and the 3 from the 13 below. Those covered numbers are all you need.

First add 15 + 3 = 18

Add a zero behind it (multiply by 10) to get 180.

Multiply the covered lower 3 x the single digit above it the "5" (3x5= 15)

Add 180 + 15 = 195.

Finger Math: 9X Rule

To multiply by 9,try this:

(1) Spread your two hands out and place them on a desk or table in front of you.

(2) To multiply by 3, fold down the 3rd finger from the left. To multiply by 4, it would be the 4th finger and so on.

(3) the answer is 27 ... READ it from the two fingers on the left of the folded down finger and the 7 fingers on the right of it.

This works for anything up to 9x10!

Square 2 Digit Number: UP-DOWN Method

Square a 2 Digit Number, for this example 37:

Look for the nearest 10 boundary

In this case up 3 from 37 to 40.

Since you went UP 3 to 40 go DOWN 3 from 37 to 34.

Now mentally multiply 34x40

The way I do it is 34x10=340;

Double it mentally to 680

Double it again mentally to 1360

This 1360 is the FIRST interim answer.

37 is "3" away from the 10 boundary 40.

Square this "3" distance from 10 boundary.

3x3=9 which is the SECOND interim answer.

Add the two interim answers to get the final answer.

Answer: 1360 + 9 = 1369

With practice this can easily be done in your head.

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