Updated on December 6, 2012

How Can I make the Multiplication of Teen Numbers Mental Math?

One of the Challenge Activities that I give my upper level math students that have mastered their multiplication facts is to learn and understand their 11 - 20 facts. These facts have a trick that you can easily learn and will allow you to do more mental math and less time with long multiplication facts.

Learn the Easy Trick

First step is to always have the larger number on top. This is essential for the problem to be able to work. The example problem I am going to do today I put the 18 on top and the 14 on the bottom.

The next step is know as the Africa step. Essential what you are doing is drawing a shape of Africa over the problem in order to know what numbers you need to use. You can also look at it as using the first two digits on the top number and the last digit on the bottom number.

So we are going to use 18 & 4. We are going to ADD these numbers together. I know this seems like an awkward step to add when we need to multiply but trust me it will work.

18 +4 = 22 - Using 22 we are going to add a zero to the end to make the number 220. This number will be part of our final answer.

Easy Trick to Multiply Teens

The next step require us to look at just the last two digit of both of the numbers. In this case the 8 and the 4.

We are going to multiply these two numbers. 8 x 4 = 32

220 + 32 = 252

I checked my answer on the calculator to ensure the correct answer.

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• akamewane

5 years ago

cool

• Jj

5 years ago

cool

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