Divisibility Rules

Dividing by 3 Add up the digits: if the sum is divisible by three, then the number is as well. Examples: 111111: the digits add to 6 so the whole number is divisible by three. 87687687. The digits add up to 57, and 5 plus seven is 12, so the original number is divisible...

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Short-cut to find square of any number

Eg 1) 362    Step 1: Choose the base number.36 is closer to 40. Therefore 40 is the base number. Step 2:Find how much the given number is more or less of the base number.36 is  -4 of 40. Step 3: Square the difference. (-4)² =16. Step 4: Add the difference to...

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A Mind-Blowing 3-Digit Number Squaring Shortcut

Now You Can Square Any Number Whose Tens Digit is 5 Without Using a Calculator To begin with, I shall first introduce my canny friend, the Rules. It will be necessary to examine these rules in order to make your way about this method simpler. Since the tens digit has already...

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Multiply Up to 20X20 In Your Head

In just FIVE minutes you should learn to quickly multiply up to 20x20 in your head.  With this trick, you will be able to multiply any two numbers from 11 to 19 in your head quickly, without the use of a calculator. I will assume that you know your...

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Divisibility by prime numbers under 50

A number is divisible by 2 if its last digit is also (i.e. 0,2,4,6 or 8). A number is divisible by 3 if the sum of its digits is also. Example: 534: 5+3+4=12 and 1+2=3 so 534 is divisible by 3. A number is divisible by 5 if the last digit is 5 or 0. Most people...

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Squaring numbers ending in 1

Here’s a simple trick to square any number that ends in 1. Subtract 1 from the number. Square the difference. (Squaring of such number is easy as it ends in ‘0’) Add the difference twice to its square. Add 1. Example: If the number to be squared is...

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Divisibility Rules

Dividing by 3 Add up the digits: if the sum is divisible by three, then the number is as well. Examples: 111111: the digits add to 6 so the whole number is divisible by three. 87687687. The digits add up to 57, and 5 plus seven is 12, so the original number is divisible...

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