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