The Magic of Maths
Some extra details to go with the Murderous Maths magic book
THE CALENDAR TRICK
Special thanks to MAX from Illinois, USA who found an ERROR on page 19 of the updated edition. The rectangle should measure 5x4, and go around any set of 20 numbers on the calendar as shown.
To get the total of all 20 numbers, add the smallest number to the biggest number and multiply by 10.
Here we get (4+29) x10 = 330.
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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.
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Instructions:
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We'll "think" of number 7
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Now we'll use x.
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Think of a number
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7
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x
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Add 2
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9
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x + 2
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Multiply by 5
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45
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5(x+2) = 5x +10
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Add 3
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48
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5x + 13
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Multiply by 2
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96
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10x + 26
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Add 4
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100
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10x + 30
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Divide by 10
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10
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x + 3
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Take away the number you first thought of
and the answer will always be 3!
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10 - 7 =3
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x + 3 - x = 3
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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...?
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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.
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This is what Pongo told Veronica to do:
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Age = 17
Weight = 55
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Age = A
Weight = B
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Put 13 into the calculator
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13
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13
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Multiply by weight
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13 x 55 = 715
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13B
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Subtract 13
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702
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13B-13
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Multiply by 7
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4914
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91B-91
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Subtract 10
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4904
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91B - 101
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Add the age
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4904 + 17
= 4921
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91B + A - 101
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Multiply by 11
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54131
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1001B + 11A - 1111
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Take away the age and
take away the weight
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54131 -17 -55
=54059
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1000B + 10A -1111
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Add 101
then show the answer!
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54160
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1000B + 10A -1010
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To find the age and weight, take the zero off the answer (i.e. divide by 10)
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5416
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100B + A - 101
=100(B-1) + (A-1)
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To find A , add 1 to the last two digits
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16 + 1 = 17
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(A-1) + 1 = A
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To find B, add 1 to the first digits
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54 + 1 = 55
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(B-1)* + 1 = B
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* when the last two digits are removed, the 100(B-1) gets divided by 100 leaving just (B-1)
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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!
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The Seven Cards of Devious Trickery
Click the cards to see how Riverboat Lil does this same trick, and find out how it works!
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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
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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
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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
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LINKS:
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