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Number Tricks

Mental Shortcuts or Mentally Efficient Maths


For these activities have the students use a pencil or pen and paper to write down each step.
PROBLEM A

Have student choose any number and to add 9 to it. (It would be easier if they pick small numbers to do this.)

Have student multiply this number by 2.

Subtract 4.

Divide the remainder by 2.

Subtract the number first chosen.


PROBLEM B

Tell a student to choose any number and add 5 to it.

Multiply this result by 3.

Subtract 9.

Divide the remainder by 3.

Subtract the number that was first selected.

Now, you can give the student the result.


PROBLEM C

Ask a student to select two numbers, each of which is less than 10.

Tell the student to choose either of the numbers and to multiply it by 5.

Tell the student to add 7 to this result.

Multiply the resulting sum by 2.

Add it to the other number that was first selected.

Have the student tell you the result.

Now you can tell the student the two numbers that were first chosen and also the one which was multiplied by 5.


ANSWERS

PROBLEM A
The result will always be 7 no matter what number was chosen.

PROBLEM B
The result will always be 2.

PROBLEM C
From whatever result the student gives you, subtract 14. You will then have a number of two figures. The two figures of this number are the two numbers which the person selected and the figure in tens place, that is the left-hand figure, is the one which the student multiplied by 5.

For example, if the student says, My result is 88, you subtract 14 from 88. The result is 74. The two numbers which the student chose were, therefore, 7 and 4, and, since 7 is the tens figure of 74, it is the number which he multiplied by 5.


Finding Someones Age

Ask the person to multiply the first number of his or her age by 5.

Tell them to add 3.

Now tell them to double this figure.

Finally, have the person add the second number of his or her age to the figure and have them tell you their answer.

Deduct 6 and you will have their age.


YOUR AGE - YOUR AGE - YOUR AGE

Multiply your age by 7

Now multiply it by 1,443

What do you get?


Number Trick Links

Briston Number Tricks
How to do a Math Magic Trick

Do Speedy Math in Your Head

Mathematical magic tricks for kids Division by Zero


Birthday Numbers and Tricks

Magical Calculator Birthday Math Trick
Guess My Birthday
Age Calculator at Math Cats

Mental Math Tricks to Impress Your Friends

MathMagic Card Trick

Mental Math

Mental Math - Wikibooks, collection of open-content textbooks

Mental Math

Impress your friends with mental Math tricks Fun Math Blog

Strategies for Mental Math

Calculation Tools and Aids - Mental Math

Mental Math Strategies Printable (4th Grade) - TeacherVision


My Math Pages