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Friday, December 15, 2017
Article written by Joe Pagano

Teach Your Kids Arithmetic - The Teenage Number Trick

So you think you can’t do math in your head, huh? Well, it all depends on how you do such math. After all, if given the right tools to do a job, then the job comes out right often enough. So why fret over the small stuff? Too often mathematics is made out to be some mysterious subject, only to be mastered by an esoteric group of like-minded nerds who co-habitate in some forsaken land. Well, that’s not the case and one of the things I’m fond of doing is debunking this oft-held yet misguided notion.

Number theory is a branch of mathematics which treats of numbers and the various properties that come out of working with these most interesting mathematical entities. All mathematics has its roots in numbers, for without a start in the domain of these “creatures,” mathematics would never build and ramify into its many distinct branches. Indeed just the knowledge and in-depth understanding of those curious numbers we call primes form the linchpin of internet security and moreover internet commerce. Yes, that’s right. Without a knowledge of prime numbers (the primes, by the way, are numbers which are only divisible by the number 1 and the number itself; thus 3 and 5 are primes because they can only be divided evenly by 1 and themselves), secure internet transactions would not be possible.

Thus understanding numbers and how to work with them via arithmetical operations form the foundations for all of mathematics. In this article, I want to give you a tool that you can pass to your children. This tool will give them the ability to multiply what I call any two “teenage numbers.” Teenage numbers are simply the numbers from 13-19—teenagers. (The method also works if we include 11 and 12, but for these poor souls the name of the method doesn’t apply as they are “pre-teens.” The method I present here is actually a default case of the 2 by 2 Cross Multiplication Technique which I teach in my Wiz Kid series. For more information on this go to my website and contact me directly.)

At any rate, the way to obtain the product from multiplying any two “teenage numbers” is as follows, in which we use 13 x 15 as the model:

Add the two digits in the ones columns of the numbers. Thus 3 + 5 = 8. Add this to 10 and add a 0 to the result. Thus 10 + 8 = 18, and 18 with a 0 added to the end of it is 180. Multiply the digits in the ones column from the two numbers and add the result to the previous step. You now have your answer. Thus 3 x 5 = 15 and 180 + 15 = 195. That’s it. With a little practice you can beat the calculator every time. Guaranteed.

Let’s try one more and you be the judge. Take 14 x 18. Now 4 + 8 = 12; 12 + 10 = 22 and 22 with a 0 at the end is 220; 8 x 4 = 32 and 220 + 32 = 252. Presto! Imagine how your children’s teachers will react when your eight and nine year olds are doing this in math class. I think the method speaks for itself. Till next time, happy multiplying.
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