# How to Multiply by 11

Instructor: Mary Beth Burns

Mary Beth has taught 1st, 4th and 5th grade and has a specialist degree in Educational Leadership. She is currently an assistant principal.

There are times in math class, as well as your everyday life, where you will have to know how to multiply by 11. Come and learn some tricks as well as some mental math strategies that will make this easier.

## Multiplying by 11

Even though it is your birthday, you are making boxes of chocolate chip cookies for your friends as favors for coming to your birthday party. Each one of the boxes can hold four chocolate chip cookies. If you are inviting 11 friends to your birthday party, how many cookies will you need to make?

Well, you could try to solve this with repeated addition, like this:

4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 4 = 44

Wow, adding four to itself eleven times takes forever! It is also very easy to make a mistake this way. This is where multiplication comes in. Multiplication is an easier solution to repeated addition. It comes in handy when you are having to work with groups, like the boxes of chocolate chip cookies. The equation would look like this:

11 x 4 = 44

Sometimes, memorizing multiplication facts can be challenging. Let's explore other strategies that you can use when multiplying by 11.

## Single-Digit Trick

The fascinating thing about multiplying by 11 is the pattern that arises when multiplying 11 by single-digit numbers. See if you can find the pattern below:

1 x 11 = 11

2 x 11 = 22

3 x 11 = 33

4 x 11 = 44

5 x 11 = 55

6 x 11 = 66

7 x 11 = 77

8 x 11 = 88

9 x 11 = 99

Did you see the pattern? You just copy the multiplicand, which are the numbers (1-9) onto the number of ones in the multiplier, which is 11, to get the product, or the answer.

## Double-Digits Trick

Let's go back to the chocolate chip cookie box problem. What if the boxes could fit 23 chocolate chip cookies? Well, the single-digit trick does not apply when multiplying 11 by a double-digit number. However, not to fret - there is still a mental math trick that works! Here is what you would do:

The number 23 has the numbers 2 and 3, which will be the first and last number of your product.

2 _____ 3

In order to find the middle number, just add the two numbers together.

2 + 3 = 5

The answer to 23 x 11 = 253.

Cool, huh?!

Note, this only works if the numbers add up to nine or less. For example, if you wanted to multiply 86 x 11, this would not work since 8 + 6 = 14, which is greater than nine.

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