In just TWO minutes you should learn to quickly multiply 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!
Three smart maths tricks made easy to multiplication
(1)
Multiplying by 9, or 99, or 999
Multiplying by 9 is really multiplying by 10-1.
So, 9×9 is just 9x(10-1) which is 9×10-9 which is 90-9 or 81.
Let’s try a harder example: 46×9 = 46×10-46 = 460-46 = 414.
One more example
68×9 = 680-68 = 612.
To multiply by 99, you multiply by 100-1.
So, 46×99 = 46x(100-1) = 4600-46 = 4554.
Multiplying by 999 is similar to multiplying by 9 and by 99.
38×999 = 38x(1000-1) = 38000-38 = 37962.
(2)
Multiplying by 11
To multiply a number by 11 you add pairs of numbers next to each other, except for the numbers on the edges.
Let me illustrate:
To multiply 436 by 11 go from right to left.
First write down the 6 then add 6 to its neighbor on the left, 3, to get 9.
Write down 9 to the left of 6.
Then add 4 to 3 to get 7.
Write down 7.
Then, write down the leftmost digit, 4.
So, 436×11 = is 4796.
Let’s do another example
3254×11.
The answer comes from these sums and edge numbers:
(3)(3+2)(2+5)(5+4)(4) = 35794.
One more example,
This one involving carrying: 4657×11.
Write down the sums and edge numbers: (4)(4+6)(6+5)(5+7)(7).
Going from right to left we write down 7.
Then we notice that 5+7=12.
So we write down 2 and carry the 1.
6+5 = 11, plus the 1 we carried = 12.
So, we write down the 2 and carry the 1.
4+6 = 10, plus the 1 we carried = 11.
So, we write down the 1 and carry the 1.
To the leftmost digit, 4, we add the 1 we carried.
So, 4657×11 = 51227 .
(3)
Multiplying by 5, 25, or 125 Multiplying by 5 is just multiplying by 10 and then dividing by 2.
Note: To multiply by 10 just add a 0 to the end of the number.
12×5 = (12×10)/2 = 120/2 = 60.
Another example:
64×5 = 640/2 = 320.
And, 4286×5 = 42860/2 = 21430.
To multiply by 25 you multiply by 100 (just add two 0’s to the end of the number) then divide by 4, since 100 = 25×4.
Note: to divide by 4 you can just divide by 2 twice, since 2×2 = 4.
64×25 = 6400/4 = 3200/2 = 1600.
58×25 = 5800/4= 2900/2 = 1450.
To multiply by 125, you multipy by 1000 then divide by 8 since 8×125 = 1000.
Notice that 8 = 2×2x2.
So, to divide by 1000 add three 0’s to the number and divide by 2 three times.
32×125 = 32000/8 = 16000/4 = 8000/2 = 4000.
48×125 = 48000/8 = 24000/4 = 12000/2 = 6000.
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!
Three smart maths tricks made easy to multiplication
(1)
Multiplying by 9, or 99, or 999
Multiplying by 9 is really multiplying by 10-1.
So, 9×9 is just 9x(10-1) which is 9×10-9 which is 90-9 or 81.
Let’s try a harder example: 46×9 = 46×10-46 = 460-46 = 414.
One more example
68×9 = 680-68 = 612.
To multiply by 99, you multiply by 100-1.
So, 46×99 = 46x(100-1) = 4600-46 = 4554.
Multiplying by 999 is similar to multiplying by 9 and by 99.
38×999 = 38x(1000-1) = 38000-38 = 37962.
(2)
Multiplying by 11
To multiply a number by 11 you add pairs of numbers next to each other, except for the numbers on the edges.
Let me illustrate:
To multiply 436 by 11 go from right to left.
First write down the 6 then add 6 to its neighbor on the left, 3, to get 9.
Write down 9 to the left of 6.
Then add 4 to 3 to get 7.
Write down 7.
Then, write down the leftmost digit, 4.
So, 436×11 = is 4796.
Let’s do another example
3254×11.
The answer comes from these sums and edge numbers:
(3)(3+2)(2+5)(5+4)(4) = 35794.
One more example,
This one involving carrying: 4657×11.
Write down the sums and edge numbers: (4)(4+6)(6+5)(5+7)(7).
Going from right to left we write down 7.
Then we notice that 5+7=12.
So we write down 2 and carry the 1.
6+5 = 11, plus the 1 we carried = 12.
So, we write down the 2 and carry the 1.
4+6 = 10, plus the 1 we carried = 11.
So, we write down the 1 and carry the 1.
To the leftmost digit, 4, we add the 1 we carried.
So, 4657×11 = 51227 .
(3)
Multiplying by 5, 25, or 125 Multiplying by 5 is just multiplying by 10 and then dividing by 2.
Note: To multiply by 10 just add a 0 to the end of the number.
12×5 = (12×10)/2 = 120/2 = 60.
Another example:
64×5 = 640/2 = 320.
And, 4286×5 = 42860/2 = 21430.
To multiply by 25 you multiply by 100 (just add two 0’s to the end of the number) then divide by 4, since 100 = 25×4.
Note: to divide by 4 you can just divide by 2 twice, since 2×2 = 4.
64×25 = 6400/4 = 3200/2 = 1600.
58×25 = 5800/4= 2900/2 = 1450.
To multiply by 125, you multipy by 1000 then divide by 8 since 8×125 = 1000.
Notice that 8 = 2×2x2.
So, to divide by 1000 add three 0’s to the number and divide by 2 three times.
32×125 = 32000/8 = 16000/4 = 8000/2 = 4000.
48×125 = 48000/8 = 24000/4 = 12000/2 = 6000.
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