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ARITHMATIC TRICKS

Added on: 22nd Oct 2015

 

 

MULTIPLY BY 5

math tricks

To do this quickly, divide by 2 and then multiply by 10

 

 

MULTIPLY BY 4

math tricks

This may seem obvious but to do this in your head, just

double twice. Some people do this intuitively and others don’t.

 

 

HAILSTONE NUMBERS

math tricks

Start with a random number. If it is even then divide by 2. If it is

odd, multiply by 3 and add 1. If you keep going you will find that no

matter where you started you will eventually hit 1. Like hailstones,

the number will go up, and inevitably come back down.

This is an example with 7:

7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1

 

 

MULTIPLE OF 3

math tricks

Your math’s teacher may never have told you this but you can

check if a number is a multiple of 3 simply by checking whether

the sum of its digits is a multiple of 3.

 

 

YOU CAN'T JUST PUT 10% BACK

math tricks

If your boss ever tells you that he is going to cut your salary by 10% but

let you work 10% more to make up for it, don’t do it! Let’s say you made

£10 per hour. 10% off would be £9 per hour. Adding 10% back would

be £9.90. Be careful what the percentage refers to!

 

 

ALL THE SQUARES

math tricks

You can get every square number by adding up the odd numbers.

Here’s the start:

1 = 1 x 1, 1 + 3 = 4 = 2 x 2, 1 + 3 + 5 = 9 = 3 x 3

 

 

"MIND READING"

math tricks

Choose a single digit number. Multiply it by 9. If the result has 2

digits then add them together. Subtract 5. Change your

number into a letter based on this pattern:

A=1, B=2, C=3…

Think of a country that begins with your letter.

Think of an animal that begins with the last letter of that country.

You most likely chose to put a kangaroo in Denmark.

 

 

THE MIRROR

math tricks

Any three digit number multiplied by 1001 will give that number

twice. 456 x 1001 is 456,456.

 

 

THE PERCENTAGE TRICK

math tricks

x% of y = y% of x. An example:

20% of 40 = 40% of 20

 


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