Multiplication tricks
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 fromright 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 leftmostdigit, 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 carrythe 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 tothe 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 multiplyby 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 your 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’sto 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.
4. Multiplying together two numbers that differ by a small even number
This trick only works if you’ve memorized or can quickly calculate the squares of numbers. If you’re able to memorize some squares and use the tricks described laterfor some kinds of numbers you’ll be able to quickly multiply together many pairs of numbers that differ by 2, or 4, or 6.
Let’s say you want to calculate 12×14.
When two numbers differ by two their product is always the square of the number in between them minus 1.
12×14 = (13×13)-1 = 168.
16×18 = (17×17)-1 = 288.
99×101 = (100×100)-1 = 10000-1 = 9999
If two numbers differ by 4 then their product is the square of the number in the middle (the average of the two numbers) minus 4.
11×15 = (13×13)-4 = 169-4= 165.
13×17 = (15×15)-4 = 225-4= 221.
If the two numbers differ by 6then their product is the square of their average minus 9
12×18 = (15×15)-9 = 216.
17×23 = (20×20)-9 = 391.
5. Squaring 2-digit numbers that end in 5
If a number ends in 5 then itssquare always ends in 25. Toget the rest of the product take the left digit and multiplyit by one more than itself.
35×35 ends in 25. We get therest of the product by multiplying 3 by one more than 3. So, 3×4 = 12 and that’s the rest of the product. Thus, 35×35 = 1225.
To calculate 65×65, notice that 6×7 = 42 and write down4225 as the answer.
85×85: Calculate 8×9 = 72 and write down 7225.
6. Multiplying together 2-digit numbers where the first digits are the same and the last digits sum to 10
Let’s say you want to multiply42 by 48. You notice that the first digit is 4 in both cases. You also notice that the other digits, 2 and 8, sum to 10. You can then use this trick: multiply the first digit byone more than itself to get the first part of the answer and multiply the last digits together to get the second (right) part of the answer.
An illustration is in order:
To calculate 42×48: Multiply 4by 4+1. So, 4×5 = 20. Write down 20.
Multiply together the last digits: 2×8 = 16. Write down 16.
The product of 42 and 48 is thus 2016.
Notice that for this particular example you could also havenoticed that 42 and 48 differ by 6 and have applied technique number 4.
Another example: 64×66. 6×7= 42. 4×6 = 24. The product is 4224.
A final example: 86×84. 8×9 =72. 6×4 = 24. The product is 7224
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 fromright 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 leftmostdigit, 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 carrythe 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 tothe 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 multiplyby 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 your 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’sto 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.
4. Multiplying together two numbers that differ by a small even number
This trick only works if you’ve memorized or can quickly calculate the squares of numbers. If you’re able to memorize some squares and use the tricks described laterfor some kinds of numbers you’ll be able to quickly multiply together many pairs of numbers that differ by 2, or 4, or 6.
Let’s say you want to calculate 12×14.
When two numbers differ by two their product is always the square of the number in between them minus 1.
12×14 = (13×13)-1 = 168.
16×18 = (17×17)-1 = 288.
99×101 = (100×100)-1 = 10000-1 = 9999
If two numbers differ by 4 then their product is the square of the number in the middle (the average of the two numbers) minus 4.
11×15 = (13×13)-4 = 169-4= 165.
13×17 = (15×15)-4 = 225-4= 221.
If the two numbers differ by 6then their product is the square of their average minus 9
12×18 = (15×15)-9 = 216.
17×23 = (20×20)-9 = 391.
5. Squaring 2-digit numbers that end in 5
If a number ends in 5 then itssquare always ends in 25. Toget the rest of the product take the left digit and multiplyit by one more than itself.
35×35 ends in 25. We get therest of the product by multiplying 3 by one more than 3. So, 3×4 = 12 and that’s the rest of the product. Thus, 35×35 = 1225.
To calculate 65×65, notice that 6×7 = 42 and write down4225 as the answer.
85×85: Calculate 8×9 = 72 and write down 7225.
6. Multiplying together 2-digit numbers where the first digits are the same and the last digits sum to 10
Let’s say you want to multiply42 by 48. You notice that the first digit is 4 in both cases. You also notice that the other digits, 2 and 8, sum to 10. You can then use this trick: multiply the first digit byone more than itself to get the first part of the answer and multiply the last digits together to get the second (right) part of the answer.
An illustration is in order:
To calculate 42×48: Multiply 4by 4+1. So, 4×5 = 20. Write down 20.
Multiply together the last digits: 2×8 = 16. Write down 16.
The product of 42 and 48 is thus 2016.
Notice that for this particular example you could also havenoticed that 42 and 48 differ by 6 and have applied technique number 4.
Another example: 64×66. 6×7= 42. 4×6 = 24. The product is 4224.
A final example: 86×84. 8×9 =72. 6×4 = 24. The product is 7224