Impress your friends with your Math skill and other mental Math tricks.

Multiply 2 digit numbers by 11

The trick is to add the two-digit numbers together and insert the sum in between the two digits.  Don’t forget to carry over if it is required.  With a little practice, you will soon be able to solve these types of problems fast.

Here's an example:

81 x 11

8 + 1 = 9, so simply insert “9” between “8” and “1”:

891

Multiply 3 digit numbers by 11

To multiply 321 by 11 go from right to left.

First write down the 1 then add 1 to its neighbor on the left, 2, to get 3.

Write down 3 to the left of 1.

Then add 3 to 2 to get 5. Write down 5.

Then, write down the leftmost digit, 3.

So, 321 ×11 = 3531.

Multiplying by 9, or 99, or 999

Multiplying by 9 is really multiplying by 10-1.

Here's how you do it:

1.)   8×9 is just 8x(10-1) which is 8×10-8 which is 80-8 or 72.

 2.)  23×9 = 23 × 10-23 = 230-23 = 207.

 

Multiplying by  99, you multiply by 100-1.

54 × 99 = 54x(100-1) = 5400 – 54 = 4554.

 

Multiplying by  999, you multiply by 1000-1.

38×999 = 38x(1000-1) = 38000-38 = 37962.

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.

14×5 = (14×10)/2 = 140/2 = 70.