Math Trick - How to Determine If A Number Is Divisible by 7 In Your Head

This video shows how to determine if a number is divisible by 7 fast.

[asdfghyter]

for the first example you can see that it's divisible by 7 even quicker, just by looking at it, since the first two digits are 21=3*7 and the last digit is 7, so the whole number is 31*7

[Llewkcor]

Good to know. And if the number ends in 0 just add 7 and then test.

[tamirerez]

The big question is
WHY THIS METHOD IS WOKING??
What is the logic ??