Divisibility by 7 - A Math Hack

Cool little math hack from John D. Cook that let’s you find if any number is divisible by 7.

Here’s the trick. Remove the last digit from the number, double it, and subtract it from the first part of the number. Do this repeatedly until you get something you recognize as being divisible by 7 or not.

For example, if we want to check 824. Pull off the 4 so you have two numbers 82 and 4. Double 4, leaving you with 8. 82-8 = 74. 74 isn’t divisble by 7, so 824 is not divisble by 7.

Magic Hands!

Divisibility by 7 — The Endeavour

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