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Jul27

Mental Math: Multiplying by 11

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In this post, we are going to learn a mental math strategy for multiplying by 11 using a simple method.

Mental Math Strategy 1: Multiplying a 2-digit number by 11

It’s only a matter of adding the number’s two digits and then putting the result in between the two numbers. Let’s take a look at some examples:

24 x 11

The number’s first digit is 2 and the second digit is 4.

24 x 11 = 2 _ 4

The space is where the sum of the two digits will go:

2 + 4 = 6 

24 x 11 = 2 ( 2 + 4 ) 4 = 264

Let’s take a look at another example:

75 x 11 = 7 ( 7 + 5 ) 5 = 7 (12 ) 5

In this example, the sum of the two digits is greater than 9, so we need to add the 10s-place digit to the number on the left, leaving the other values as they are.

( 7 + 1 ) 2 5 = 825

75 x 11 = 825

Mental Math Strategy 2: Multiplying a number with more than 2 digits by 11

The mechanics are the same: The first and last digits of the number go on the ends. The intermediate numbers are the sum of the digits that are next to each other. It makes more sense with an example:

248 x 11 = 2  ( 2 + 4 ) + ( 4 + 8 ) 8 = 2 ( 6 ) ( 12 ) 8

We separate the 12 by adding the 1 to the digit on its left.

2 ( 6 + 1 ) ( 2 ) 8 = 2728

248 x 11 = 2728

If you liked this post about multiplying by 11 and want to learn more tricks with the number 11, check out divisibility criteria by 11.

And If you want to learn much more elementary math, try Smartick for free!

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