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DIVISIBILITY OF NUMBERS WITHOUT A CALCULATOR

Here are some tips on how to go about calculations without the use of calculators. Some of you might be familiar with them while other may not.


DIVISIBILITY OF NUMBERS WITHOUT A CALCULATOR
Number
Divisibility
Example(specific)
1
All number are divisible by 1, it is no secret

2
All number whose last digit are even numbers are divisible by 2

3
If the sum of digits(numbers that make up the number) is divisible by 3.
Examples 123,144,135 are all divisible by 3.
123/3=41,144/3 = 48, 135/3=45
4
4 is a multiple of 2, if a number can be divided by 2 twice, then it is divisible by 4.

5      quite simple
If the last digit of a number is digit number is 5, then the number is divisible by 5
15, 25, 305 e.t.c
6
If the last digit is an even number and the sum of the digit is divisible by 3
18,12,54 e.t.c.
7     Quite easy
If the last digit is added to 3 multiply the front digits and gives a number divisible by 7
14 is divisible by 7 because  (1x3) + 4 = 7
21 is divisible by 7
(2x3)+1 = 7
84 is also divisible by 7
(8x3)+4 = 28
154 is also divisible by 7 because
(15x3)+4 = 49
8   
8 is a multiple of 2 so it is a no brainer.

9
Though 9 is a factor of 3 as well. But if the sum of the digits adds up to multiples of 9, then it is divisible by 9.
For instance 18 is 1+8=9,
27 is 2+7 = 9, 117 is 1+1+7=9.
These are all divisible by 9.

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