Let me explain this rule by taking examples
If you had 343, you would double the last digit to get six, and subtract that from 34 to get 28.
If you get an answer divisible by 7 (including zero), then the original number is divisible by
seven. If you don't know the new number's divisibility, you can apply the rule again
EXAMPLE.
Example -:
623: 62 - (3 × 2) = 56 = 7 × 8
483: 48 − (3 × 2) = 42 = 7 × 6
MULTIPLICATION OF ANY TWO NUMBERS, LIES BETWEEN 11 AND 19
Let me explain this rule by taking examples
13*19 = (13+9)*10 + (3*9) = 220 + 27 = 247
Means add first number and last digit of the second number take zero in the third place of this number then add product of last digit of the two numbers in it.
EXAMPLE.
18*14 = (18+4)*10 + (8*4) = 220 + 32 = 252
If a number is divisible by two it will end in an even number or a 0.
EXAMPLE.
For example
28964
given number end with 2 then it will be divisible by 2.
and
89450
given number end with 0 it will also be divisible by 2.
If one is allowed as a prime, then any number could be written as a product of primes in many ways.
EXAMPLE.
24 = 1*2*2*2*3
or 24 = 1*1*2*2*2*3
or 24 = 1*1*1*1*1*2*2*2*3
The fact that factoring into primes can only be done in one way is important in mathematics.
If a number is divisible by three the sum of its digits will be divisible by 3.
EXAMPLE.
372 = 3 + 7 + 2 = 12
A corollary of this is that any number made by rearranging the digits of a number divisible by 3 will also be divisible by 3.
EXCEPTIONAL DIVISION RULE FOR 4
If a number is divisible by four the last two digit are divisible by 4 (or are zeros).
EXAMPLE.
45624
last two digit is 24
24 is divisible by 4 then 45624 is divisible by 4.
Reference:M4maths.com
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