There are times when the multiplication algorithm
gives you some shortcut multiplications if you just inspect what you are doing.
Here is a neat little multiplication shortcut to perform various
multiplications with the numbers 21, 31,

*41,**51 and so on.*
To multiply by 21: Double the number, append a ‘0’
after that and add the original number.

For example: To multiply 37 by 21,

Double 37 yields 74, append a 0 to get 740; and
then add the original number 37 to get 777.

To multiply by 31: Triple the number, attach a ‘0’
after that and add the original number.

For example: To multiply 43 by 31,

Triple 43 yields 129, attach a 0 to get 1,290; and
then add the original number 43 to get 1,333.

To multiply by 41: Quadruple the number, put a ‘0’
after that and add the original number.

For example: To multiply 47 by 41,

Quadruple 47 yields 188, put a 0 to get 1,880; and
then add the original number 47 to get 1,927.

Extend the rule further to other numbers.

**If the multiplicand is a big one and to multiply mentally follow the below method:**

**Multiplying by 21**

Rule: The units (right) digit of the
answer is the units digit of the given number. The tens (2

^{nd}from right) digit of the answer will be twice the units digit plus the 2^{nd}-right digit of the given number. The 3^{rd}-right digit of the answer will be twice the second digit plus the 3^{rd}-right digit of the given number. Continue the process for the rest of digits of the given number. For the leftmost digit(s) of the answer simply double the leftmost digit of the given number. Whenever a sum is a two-digit number, record its units digit and add the tens digit to the left answer digit.
This rule is very much like the one for
multiplying by 11. In fact, since 21 is the sum of 11 and 10, it does belong to
the same family of short cuts.

As an example, we shall multiply 5,392
by 21.

The unit’s digit of the answer is the
unit’s digit of the given number, 2.

The tens digit of the answer is
obtained by adding the tens digit of the given number to twice the units digit
of the given number.

Record the 3; carry the 1 to the left.

The next digit is obtained by adding 3
to twice 9 plus carried 1.

(2 x
9) + 3 + 1 = 22

Record the 2 and carry the 2 to the
left.

Next, add the first digit of the given
number, 5, to twice the second digit, 3 plus carried 2.

(2 x 3) + 5 + 2 = 13

Record the 3 and carry the 1 to the
left.

The leftmost digit(s) of the answer
will be equal to twice the first digit of the given number plus carried 1.

(5 x 2) + 1 = 11.
The product is therefore, 5,392 x
21 =
113,232

Extend the rule further to other numbers i.e. 31,
41, 51 and so on.

very fantastic technique

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