Well, this is fascinating.
It's a graph by David Wilson to determine if something is divisible by 7. Check out the link to Tanya Khovanova's blog to see how it works.
"Write down a number n. Start at the small white node at the bottom of the graph. For each digit d in n, follow d black arrows in a succession, and as you move from one digit to the next, follow 1 white arrow.
For example, if n = 325, follow 3 black arrows, then 1 white arrow, then 2 black arrows, then 1 white arrow, and finally 5 black arrows.
If you end up back at the white node, n is divisible by 7."
The comment section over there is just as fascinating.
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