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.
That is It! Wasn't that easy? Practice it on paper first!
Multiply By 5
To quickly multiply by 5, divide the number in two and then multiply it by 10. Often this can be done quickly in your head.
The 11 Rule
You likely all know the 10 rule (to multiply by 10, just add a 0 behind the number) but do you know the 11 rule? It is as easy! You should be able to do this one in you head for any two digit number. Practice it on paper first! To multiply any two digit number by 11:
For this example we will use 54. Separate the two digits in you mind (5__4). Notice the hole between them! Add the 5 and the 4 together (5+4=9) Put the resulting 9 in the hole 594. That's it! 11 x 54=594 The only thing tricky to remember is that if the result of the addition is greater than 9, you only put the "ones" digit in the hole and carry the "tens" digit from the addition. For example 11 x 57 ... 5__7 ... 5+7=12 ... put the 2 in the hole and add the 1 from the 12 to the 5 in to get 6 for a result of 627 ... 11 x 57 = 627
Practice it on paper first!
Effect
You write down the following 8 digit number on a piece of paper:
1 2 3 4 5 6 7 9
Then ask a friend to circle one of the digits. Say that they circle number 7.
You then ask your friend to multiply the 8 digit number by 63, and magically the result ends up being:
1 2 3 4 5 6 7 9x 6 3 7 7 7 7 7 7 7 7 7
with the answer as a row of the chosen number 7.
The SecretWhen your friend circles a number, you need to multiply the chosen number by 9 in your head - if 3 was chosen you would work out 3 x 9 = 27. Then you need to ask your friend to multiply the 8 digit number by the number you have just worked out. In the case of 3 being chosen you ask your friend to multiply 12345679 by 27 and you magically get the answer 333333333.
