# A Neat Math Trick

My daughter taught me this trick today. And I thought that the teachers on Persona Paper might want to share this with their students or parents with their children.

Take any date.

For example, today is 17-12-2015

Jumble up the numbers: 21715120

Now, find the difference between the two numbers which is 4593195

Now add the digits in these numbers together (4+5+9+3+1+9+5)

The answer is 36.

Add these two digits together (3+6) and the answer is 9.

It is always 9, no matter what date you take. Try it with your date of birth or ask your students to try this with their dates of birth.

Sometimes tricks like these can liven up the classroom.

Let's take another date 01-01-2016

Jumbles up version: 61021010

Difference between two dates: 60008994

Adding digits: 36

Add these two digits together and the answer will be 9.

Aren't numbers amazing!

**Image Credit**» https://pixabay.com/en/mathematics-maths-blackboard-chalk-998343/ by ArtsyBee

## Comments

bestwriter wrote on December 17, 2015, 7:05 AM

No doubt numbers are truly amazing but will someone tell me what this is all about

MegL wrote on December 17, 2015, 7:48 AM

I must ask my sister why this works - she is a mathematician.

wolfgirl569 wrote on December 17, 2015, 9:26 AM

That does look fun for kids to do.

Rufuszen wrote on December 17, 2015, 11:22 AM

Not this particular one, but we were always playing with methods like these when I was in school

Dawnwriter wrote on December 17, 2015, 12:48 PM

bestwriter Haha, sorry, I got a bit carried away . we are always trying these maths tricks at home. Like guessing numbers or maths riddles. Just felt like sharing this amazing phenomenon.

cmoneyspinner wrote on December 17, 2015, 1:38 PM

What I like about numbers is that can not be "interpreted". The answer is the answer. That's that.

Paulie wrote on December 18, 2015, 3:28 AM

In the United States dates are expressed different from how they are in Britain. For example, December 18, 2015 is written as 12-18-2015. I did your example with dates expressed this way, and the answer as you pointed out is always 9.

Dawnwriter wrote on December 18, 2015, 3:59 AM

Paulie Working a bit more on this puzzle, we found that this hold true for any number, dates are not important. For example, 100, jumbled into 010. Difference between the two numbers is 90. Add digits 9+0=9. You can try it for any number no matter how long or short.

Dawnwriter wrote on December 18, 2015, 4:00 AM

I would love to know the explanation.

CoralLevang wrote on December 18, 2015, 11:21 AM

This is amazing. I do not know the explanation and am curious.

DWDavisRSL Do you know?

DWDavisRSL wrote on December 18, 2015, 11:42 AM

Yes, I do know why this works but I don't want to spoil the fun just yet CoralLevang

CoralLevang wrote on December 18, 2015, 11:46 AM

You could be the one to save the day here. I know there are others who will be the ones to do so, if you do not act quick enough.

DWDavisRSL wrote on December 18, 2015, 12:32 PM

The secret to this trick is transposition. When numbers are transposed (mixed up in their order) the result will always be a multiple of 9, and the sum of the digits will always be a multiple of 9. And the numbers in two digit multiples of 9 will always equal 9. Okay, CoralLevang ?

CoralLevang wrote on December 18, 2015, 12:39 PM

But why?

DWDavisRSL wrote on December 18, 2015, 4:08 PM

There is no why. The difference between two numbers that are transpositioned will always be 9 or a multiple of 9. It is just a universal truth, like the fact that the circumference of a circle divided by its diameter always equals pi, no matter the size of the circle.

CoralLevang wrote on December 18, 2015, 4:12 PM

but there has to be some sort of reasoning why or how? Isn't there? How it works?this is the kind of stuff that got me into trouble in school all the time cuz I just wanted to understand how and why because there's a story behind it all. Just like in music, when I ask how no one would tell me. They said it just is what it is. But there is a mathematical science behind the music and harmony. So, why is there not some theory behind all of this?

DWDavisRSL wrote on December 18, 2015, 6:24 PM

The theory is the Theory of Transpositional Numbers. It states that if the digits of a number are rearranged to form a new number, the difference between the two numbers will be a multiple of nine thus the sum of the digits in the difference will also equal either nine or a multiple of nine.

For example: 9,527 - 2,597 = 6,930 which is the 770th multiple of 9. The sum of 6+9+3+0 = 18 which is the second multiple of 9. 1+8 = 9.

DWDavisRSL wrote on December 18, 2015, 6:27 PM

Also referred to as Transposition Error when it happens in accounting.

bestwriter wrote on January 19, 2016, 11:35 PM

Explain, explain. How long do you want me to keep guessing. I like math quizzes and spend a lot of time whenever I get to see them