Rule for dividing by 7
Date: Thu, 1 Dec 1994 08:17:25 -0600 (CST) From: Daniel Sanfilippo Subject: Divisibility rules Many years ago, I taught from a math text that gave "divisibility rules" for the numbers one through ten. For example, "if the sum of the digits of a number is divisible by three, the number is divisible by three." I remember the rules for all the numbers except "7". I do remember that this rule was rather complicated and cumbersome, but it was still useful. I've looked in every reference book I could think of, but have had no luck. Any chance you could help me out here? It would certainly be appreciated. Dan Sanfilippo Sixth Grade Teacher Masters Elementary School Omaha, NE
Date: Thu, 01 Dec 1994 18:56:12 -0500 From: Daniel Eisenbud Organization: Geometry Forum Subject: Re: Divisibility rules I do remember the rule. To find out if a number is divisible by seven, you take the last digit, double it, and subtract it from the rest of the number. So if you had 203, you would double the last digit to get six, and subtract that to get 14. If you get an answer divisible by 7 (including zero, of course) then the original number is divisible by seven; if you don't know the new number's divisibility, you can apply the rule again. A summary of divisibility rules from 3 to 13, with explanations for why they work, can be found at http://mathforum.org/k12/mathtips/division.tips.html -Doctor Dan Eisenbud, The Geometry Forum Check out our web site! http://mathforum.org/dr.math/
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