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Short Trick for Multiplying A Number by 21, 31, 41, 51, 61, 71, 81, 91, 101, 111, ...



Tricks to Multiply any number by 21

Let us understand how it works by multiplying 63852 X 21

Step 1: Start by adding an imaginary 0(zero) after the number 638520


Step 2: Multiply the rightmost number with 2, here in case is an imaginary number i.e.

0 X 2 = 0 and add with the corresponding number from left i.e. 0 + 2 = 2.


Step 3: Take one by one numbers from left and repeat step 2 i.e. 2 X 2 = 4 and add with the corresponding number from left i.e. 4 + 5 = 9 and place the number to the left number obtained in previous step i.e. 92


Step 4: Take one by one numbers from left and repeat step 2 i.e. 5 X 2 = 10 and add with the corresponding number from left i.e. 10 + 8 = 18 and place the number to the left number obtained in previous step i.e. 892 and keep the carry 1 for the next step.


Step 5: Take one by one numbers from left and repeat step 2 i.e. 8 X 2 = 16 and add with the corresponding number from left i.e. 16 + 3 = 19 and add it with the carry from the previous step i.e. 19 + 1 = 20 and place the number to the left number obtained in previous step i.e. 0892 and keep the carry 2 for the next step.


Step 6: Take one by one numbers from left and repeat step 2 i.e. 3 X 2 = 6 and add with the corresponding number from left i.e. 6 + 6 = 12 and add it with the carry from the previous step i.e. 12 + 2 = 14 and place the number to the left number obtained in previous step i.e. 40892 and keep the carry 1 for the next step.


Step 7: Take one by one numbers from left and repeat step 2 i.e. 6 X 2 = 12 and add it with the carry from the previous step i.e. 12 + 1 = 13 and place the number to the left number obtained in previous step i.e. 1340892.


So, 63852 X 21 = 1340892 (Answer)


You may have noticed that the above example was with multiplication of 21. This method is applicable for any multiplication with number whose unit digit is 1 i.e. 21, 31, 41, 51……. Here’s an example to demonstrate.


Tricks to Multiply any number by 31

Let us understand how it works by multiplying 63852 X 31

Step 1: Start by adding an imaginary 0(zero) after the number 638520


Step 2: Multiply the rightmost number with 3, here in case is an imaginary number i.e. 0 X 3 = 0 and add with the corresponding number from left i.e. 0 + 2 = 2.


Step 3: Take one by one numbers from left and repeat step 2 i.e. 2 X 3 = 6 and add with the corresponding number from left i.e. 6 + 5 = 11 and place the number to the left number obtained in previous step i.e. 12 and keep the carry 1 for the next step.


Step 4: Take one by one numbers from left and repeat step 2 i.e. 5 X 3 = 15 and add with the corresponding number from left i.e. 15 + 8 = 23 add it with the carry from the previous step i.e. 23 + 1 = 24 and place the number to the left number obtained in previous step i.e. 412 and keep the carry 2 for the next step.


Step 5: Take one by one numbers from left and repeat step 2 i.e. 8 X 3 = 24 and add with the corresponding number from left i.e. 24 + 3 = 27 and add it with the carry from the previous step i.e. 27 + 2 = 29 and place the number to the left number obtained in previous step i.e. 9412 and keep the carry 2 for the next step.


Step 6: Take one by one numbers from left and repeat step 2 i.e. 3 X 3 = 9 and add with the corresponding number from left i.e. 9 + 6 = 15 and add it with the carry from the previous step i.e. 15 + 2 = 17 and place the number to the left number obtained in previous step i.e. 79412 and keep the carry 1 for the next step.


Step 7: Take one by one numbers from left and repeat step 2 i.e. 6 X 3 = 18 and add it with the carry from the previous step i.e. 18 + 1 = 19 and place the number to the left number obtained in previous step i.e. 1979412.


So, 63852 X 21 = 1979412 (Answer)

Similarly, using this method you can calculate the product of any number when divided by number whose ones is 1 i.e 21, 31, 41, 51, 61, ...
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