ARITHMATIC TRICKS
Added on: 22nd Oct 2015
MULTIPLY BY 5
To do this quickly, divide by 2 and then multiply by 10
MULTIPLY BY 4
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
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
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
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
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"
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
Any three digit number multiplied by 1001 will give that number
twice. 456 x 1001 is 456,456.
THE PERCENTAGE TRICK
x% of y = y% of x. An example:
20% of 40 = 40% of 20
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