How to beat a calculator adding numbers! (math tricks)

More fun with 9's

This trick is so simple and it has blown away everyone I've tried it on. I hope you have a lot of laughs over this one. This is how it works:

You challenge a friend or family member to a race adding numbers. They get to use a calculator and you don't. Here's how you set it up:

  1. Have them write a 5 digit number (minimum) on a piece of paper (tell them not to use 0 or any duplicate numbers).
  2. You take the paper and write your own five digit number underneath theirs.
  3. They write another five digit number under yours. You don't look at this number (yet).
  4. Take the paper from them and turn it over (writing down).
  5. Let them get ready with their calculator.
  6. Say "go", then turn over the paper and write the answer before they can add it up on the calculator.
  7. Oh, you want to know how to beat them? :)

Here's how:

When you write your number under theirs, make each column add up to 9. Example: they write 12345, you write (seemingly random numbers) 87654 underneath.

  12345

+87654

 (99999)

 
 
 
 

Then they write the third row of numbers under yours. Let's say they write 57263. When you write the answer, you simply start on the right, subtract a 1 from the last number in the third row (3 in this case), then copy the remaining numbers exactly, then put a 1 at the beginning of your number. So, your answer in this example would be 157262. Easy, huh?

  12345

  87654

+57263

157262

 
 
 

Try it again. They write 62891. You would write 37108. Let's say they write 12345 in the last row. The answer would be 112344.

And when they finally get it added up on the calculator, they will see that you are correct. :)

  62891

  37108

+12345

112344

 
 
 
 
 

This works on numbers with as many digits as you care to try. You can also extend the number of rows that are used.

If you use 5 rows, you must subtract 2 from the last number on the right, copy exactly the remaining numbers, then put the 2 at the beginning.

Using five rows is set up by having them write a number, then you write a number underneath that makes each column add up to 9, they write another number under yours, then you write another number underneath that makes their most recent number add up to nine, they write the final number, then you add it up.

Example:

  12345

  87654

  32895

  67104

+43972

243970

 
 

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Comments 13 comments

SimeyC profile image

SimeyC 7 years ago from NJ, USA

Should confuse a few friends out there as they try and fiugure out how you add them so quickly!


habee profile image

habee 7 years ago from Georgia

Ugh. I hate math, but you managed to make it seem almost fun! Neat hub.


jiberish profile image

jiberish 7 years ago from florida

I love it. This will be fun. Thank you!


House Of Bread profile image

House Of Bread 7 years ago

Stretching the grey matter a bit. Good one.


keira7 profile image

keira7 7 years ago

Hi Lowell`s, thanks for this very good hub. Take care.


Gicky Soriano profile image

Gicky Soriano 7 years ago from California

I'll be trying this out this trick with friends tonight. Thanks!


Bovine Currency profile image

Bovine Currency 7 years ago

Interesting. You, like I, seem to have too much time! Kidding, fun topic. I love numbers but it doesn't seem to impress cashiers and bank tellers, nor the telemarketers trying to consolidate my debts. Just a bit of a quirk I guess.

45 by 73 by 81 = 45 by 7 215 add a 0 2150 and 3 by 45 is 135 and mental add 150 and 135 like you did above so you get 2 in front 1+1 5+3 and 5 on its own 2280 by 81 is 2280 by 10 by 8 or 2280 by 10 by 10 228000 - (2300*2) would be 22 in the fron and 8000 - 5000 - 6000 or 2400 226000 - 600 would be 225400 and then of course add the 2150 and you have 225400 , that being 2256500. Now checking my calculator, and I was entirely wrong 226085. Strangely, I think to myself, you were close Cash Cow but not close enough... maybe you should go count sheep? I don't sleep. Silly me.Signing off soon, 0450 hours.


Lowell's Notes profile image

Lowell's Notes 7 years ago Author

Bovine Currency, this made my day. Thanks for making me laugh. :D

Gicky, I hope you had fun with this.

Thanks Kiera! :)

House of Bread, Great mental exercise, huh?

Jiberish, Enjoy!

Habee, Thanks!

SimeyC, This has always been a fun one.

Sorry...just now playing catch up. Thanks for all of your comments. :)


Curus Terrum profile image

Curus Terrum 7 years ago

LOL Let's try it on some of my fellow 6th graders


Lowell's Notes profile image

Lowell's Notes 7 years ago Author

Go get 'em Curus! :)


CalcGuy profile image

CalcGuy 6 years ago from Peoria

Thanks for this. Students love these kinds of tricks.


Taleb80 profile image

Taleb80 5 years ago

It is a nice trick.

Thanks for sharing.

I voted "Useful"


Trsmd profile image

Trsmd 5 years ago from India

simple logic and easy to understand it..for kids..

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