Math Tricks you can play on your friends

One Trick * Create Your Own Tricks * Beat the Calculator * Books to Borrow or Buy

A student asked me if I knew any math tricks she could pull on her friends.

Here's what I told her:

There are two different kinds of math tricks that I can think of that you might be interested in.

Tricks that you can play on your friends, and
Tricks that you can use to calculate answers

Here's one trick you can play on your friends.
Do this trick yourself, one step at a time.
Pretend that I'm telling you each step.
Do it on paper and see if I trick you:

Pick a number between 1 and 9?
Multiply it by 3.
Add 5 to that answer.
Double your answer:
Subtract four
Divide by six
Add three to your number
Subtract your original number
Think of a country that begins with that letter of the alphabet
(A=1, B=2, C=3, D=4, etc.)
Think of an animal that starts with the second letter of that country name.
Think of a color for that animal.

What did you come up with? Don't scroll down until you have tried all of this. If you didn't do the calculations, go back and do it now. You'll be glad you did.

Don't peek

Hey, I said "Do everything first."

If you don't have a color, animal and country already, go back up and do everything first.

Hey, that's cheating, do it first.

Okay, I'm reading your mind . . . here's what I think you came up with:

I think you might have come up with a Gray elephant from Denmark?

It won't work with everyone you try this on, but you'll be amazed as to how many people get this answer, especially the adults.

You can create your own math tricks like this, even "on the fly" if you use a little bit of algebra. Even if you haven't had Algebra yet, you can do it.

What you tell your friend to do: What you write down on your paper

Pick a number X

Add 7 to your number X+7

Double what you have now 2 * (X+7) = 2X + 14
(make sure you multiply both parts)

Subtract 8 2X + 14 - 8 = 2X + 6

Multiply by 3 3 * (2X + 6) = 6X + 18

Subtract 6 6X + 18 - 6 = 6X + 12

Divide by 6 (6X + 12) ÷ 6 = X + 2
(make sure you divide both parts)

Hey, now we have a single "X" in our answer.
If we subtract "his original number", we get rid of it.

Subtract your original number X + 2 - X = 2

2 is your answer. Of course, you can do this with YOUR OWN NUMBERS. Once there is no X in your formula, you know that your friend's number is the same as your number.

When I do this with kids, I like to make the answer come out to some number that I figured out in advance. With one child, I got his birthday from his mom, then made the answer come out to the "day". He was amazed when I said, "You ended up with the day that your birthday will happen."

There are also math tricks that you can do to get answers.
Learn these tricks and you can calculate faster than your friend can use a calculator.

What you want to do The Math Trick An Example

Add 9 Add 10 then subtract 1 63 + 9 = 63 + 10 - 1 = 73 - 1 = 72

Add 99 Add 100 then subtract 1 57 + 99 = 57 + 100 - 1 = 157 - 1 = 156

Add 999 Add 1000 then subtract 1 1776+999 = 1776+1000-1 = 2776-1 = 2775

Multiply two #'s that
are two apart Square the inbetween #
then subtract one. 24*26 = 25^2-1 = 625-1 = 624
11*13 = 12^2-1 = 144-1 = 143

Square a # that
ends with 5 Multiply all but last digit
by the next higher number.
That's first part of answer.
25 is the rest of answer. 35^2 ---> 3*4 = 12 ---> 1225
85^2 ---> 8*9 = 72 ---> 7225
115^2 ---> 11*12=132 ---> 13225
995^2 ---> 99*100=9900 ---> 990355

Multiply two-digit
number by 11 Write down other number
with space between digits
Add digits for middle digit
If sum > 10, carry the 1. 35*11 ---> 3_5 ---> 3+5=8 ---> 385
23*11 ---> 2_3 ---> 2+3=5 ---> 253
75*11 ---> 7_5 ---> 7+5=12 --> 825

There are many other tips out there. If you would like to get one "Beat the Calculator" tip every week in your e-mail, write to <BEATCALC@aol.com>. Here's one out of his tenth newsletter:

Multiply a two-digit number times 55? Divide the number by 2. Multiply that number by 10, write it down, multiply it by 10 again, write it down, then add those two numbers.
Example: 28x55 ---> 28/2 = 14 ---> 140 + 1400 = 1540

Eventually, if there is enough interest, maybe I can post some flash cards based on math tricks. Let me know if you are interested in seeing me invest some time in that project.

Math Tricks Books --- Borrow or Buy

You can find many more math tricks in books in your library. A very good friend of mine gave me the book Math Magic (by Scott Flansburg & Victoria Hay). She bought it at a local bookstore. You might be able to borrow it from your school library or your public library. Once you see everything it has to show, you might want to buy your own copy. You should be able to find it in bookstores in your area. Click here if you want to see its listing at AMAZON.COM, the internet bookstore.

Because I was curious, I used Amazon's search feature, doing a search on the phrase "Math Magic". It told me about a number of other books. I then searched for other interesting book titles. You should be able to find some of these books in your local libraries or bookstores. The prices listed are Amazon.com's, do not include shipping, and are included so that you can compare deals with your bookstore if you so desire. Amazon.com now has a Look Inside feature that lets you read some of the inside pages, so if you're not sure about a book, go there and read some of the content before you decide to buy. Then, when you decide to buy your own copy, you know in advance what you are getting.

Books I found at Amazon.com:

Math Magic for Your Kids : Hundreds of Games and Exercises from the Human Calculator to Make Math Fun and Easy ~ Scott Flansburg (Since it's written by the same person who wrote Math Magic, I bet it's a good book).
The Magical Math Book ~ Bob Longe, Jeff Sinclair
Short-Cut Math ~ Gerard W Kelly
How to Calculate Quickly ~ Henry Sticker
Speed Mathematics Simplified ~ Edward Stoddard

I hope this has been helpful.

If you enjoy puzzles, please visit my web pages:

<www.MAZES.com>
<www. GodLovesEveryone.org>

What you tell your friend to do:	What you write down on your paper
Pick a number	X
Add 7 to your number	X+7
Double what you have now	2 * (X+7) = 2X + 14 (make sure you multiply both parts)
Subtract 8	2X + 14 - 8 = 2X + 6
Multiply by 3	3 * (2X + 6) = 6X + 18
Subtract 6	6X + 18 - 6 = 6X + 12
Divide by 6	(6X + 12) ÷ 6 = X + 2 (make sure you divide both parts)
	Hey, now we have a single "X" in our answer. If we subtract "his original number", we get rid of it.
Subtract your original number	X + 2 - X = 2

What you want to do	The Math Trick	An Example
Add 9	Add 10 then subtract 1	63 + 9 = 63 + 10 - 1 = 73 - 1 = 72
Add 99	Add 100 then subtract 1	57 + 99 = 57 + 100 - 1 = 157 - 1 = 156
Add 999	Add 1000 then subtract 1	1776+999 = 1776+1000-1 = 2776-1 = 2775
Multiply two #'s that are two apart	Square the inbetween # then subtract one.	2426 = 25^2-1 = 625-1 = 624 1113 = 12^2-1 = 144-1 = 143
Square a # that ends with 5	Multiply all but last digit by the next higher number. That's first part of answer. 25 is the rest of answer.	35^2 ---> 34 = 12 ---> 1225 85^2 ---> 89 = 72 ---> 7225 115^2 ---> 1112=132 ---> 13225 995^2 ---> 99100=9900 ---> 990355
Multiply two-digit number by 11	Write down other number with space between digits Add digits for middle digit If sum > 10, carry the 1.	3511 ---> 3_5 ---> 3+5=8 ---> 385 2311 ---> 2_3 ---> 2+3=5 ---> 253 75*11 ---> 7_5 ---> 7+5=12 --> 825