Divisibility by 7

This test helps us in finding if a given number is divisble by 7. There are a lot of other divisibility tests spread across the web, but I found this one to be easy!. I've also listed other possible ways to find if a given number is divisible by 7 at the end of this blog.

To begin with, it is required to split a number into sections of "3" digits. This can be better understood by an example.

Take this number, 4555654. Let us now find if this number is divisible by 7.

Step - 1: Starting from unit digit, slice the given number into set of three digits. So, we get
4 555 654

Step -2: Add the even parts separately and the odd parts separately. For our example, we get
658 & 555.

Step -3: Find the difference between the two parts. In our case, it is 103.

Step -4: Find if 103 is divisible by 7. It is not. Hence the number 4555654 is not divisible by 7.
Also, the number leaves a remainder of 5 when divided by 7. (Inferred by dividing 103 by 7)

Let us do a quick cross check with the calculator.. Hurray! it is true.

Now, let us repeat the example with a really big number. 1636871900636.

Step -1: Slicing..
1 636 871 900 636

Step -2: Summing
1+871+636=1508
636+900= 1536

Step -3: Subtracting (difference) -> 28

Step - 4: Check if 28 is divisible by 7 and it is :). So, the big number 1636871900636 is divisible by 7.

So, try your hand on this! Post your comments or any other better way to do the check. Try here and here for some more ways.