New Divisibility Rule of ' 7 "

Divisibility Rule of " 7 ":

                                                                 If a number is divisible by " 7 ", The last digit of the number is doubled and subtracted from the remaining number if the obtaining number is divisible by 7 then the whole number is divisible by " 7 "

For Example: 3024

the last digit of the number is, double the number 

            i.e,4*2=8

Subtract that number from the remaining number i.e, 302 -8=294

At this we use to check directly by 7 or we can do another iteration to get direct multiple of the 7

Again from 294, the last digit is doubled and subtracted from the remaining number

29-4*2=29-8=21

on sight, we can say that 21 is divisible of 7 then the whole number is divisible by "7"

Another Example:

            Is it 398874 is divisible of 7?

Iteration 1: double the last digit and subtract form the number

                39887-4*2=39887-8=39879

Iteration 2: again do the same thing double and subtract

                  3987-9*2=3987-18=3969

Iteration 3: Repeat 

                   396-9*2=396-18=378

Iteration 4:Repeat 

                 37-8*2=37-16=21

on sight that 21 is divisible by 7 

therefore the whole number is divisible by 7