The Magic of Maths
Some extra details to go with the Murderous Maths magic book




Think of a Number: The Magic 3
How does it work?
This table shows the trick using the number 7, then we'll explain it
using algebra. Instead of a number, we use letter x!
Letter X can represent ANY number. In the end all the x's disappear, which shows that it doesn't matter which number you choose to start with.


Instructions:

We'll "think" of number 7

Now we'll use x.

Think of a number

7

x

Add 2

9

x + 2

Multiply by 5

45

5(x+2) = 5x +10

Add 3

48

5x + 13

Multiply by 2

96

10x + 26

Add 4

100

10x + 30

Divide by 10

10

x + 3

Take away the number you first thought of
and the answer will always be 3!

10  7 =3

x + 3  x = 3




Think of a Number: Think of TWO numbers!
Pongo asks Veronica to put her age and weight secretly into a calculator. Later she is allowed to "take them" away so he can't know what they are.
Or can he...?

How does it work?
This trick involves some more complicated algebra. It uses two numbers  Veronica's age and her weight. We'll call these numbers A and B. In fact the trick can work with almost ANY two numbers, so long as number A is less than 100.



This is what Pongo told Veronica to do:

Age = 17
Weight = 55

Age = A
Weight = B

Put 13 into the calculator

13

13

Multiply by weight

13 x 55 = 715

13B

Subtract 13

702

13B13

Multiply by 7

4914

91B91

Subtract 10

4904

91B  101

Add the age

4904 + 17
= 4921

91B + A  101

Multiply by 11

54131

1001B + 11A  1111

Take away the age and
take away the weight

54131 17 55
=54059

1000B + 10A 1111

Add 101
then show the answer!

54160

1000B + 10A 1010

To find the age and weight, take the zero off the answer (i.e. divide by 10)

5416

100B + A  101
=100(B1) + (A1)

To find A , add 1 to the last two digits

16 + 1 = 17

(A1) + 1 = A

To find B, add 1 to the first digits

54 + 1 = 55

(B1)* + 1 = B

* when the last two digits are removed, the 100(B1) gets divided by 100 leaving just (B1)




The Predictor Cards
Click the picture to see how to do the trick, or click the little picture
for a sheet you can print out to make the cards!


The Seven Cards of Devious Trickery
Click the cards to see how Riverboat Lil does this same trick, and find out how it works!


The Fabulous Flexagons
Click the picture to find a template you can print out to make the BABYFLEX and the MONSTERFLEX.
You can see how to make some other flexagons at
How to Make a Triflexagon


The Magic Ball
Click the picture to find a template you can print out to make the DODECAHEDRON. You can find more instructions at our dodecahedron Page

EXTRA TRICKS!
Try our Three Magical Colour Illusions!
Have you seen our Tricks and Games page?
You'll also find a few more tricks and things to do on our Teacher Page

LINKS:




