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A comment on Is this prime?. Parent comment

Re:Is this prime? by Bahamut on Tuesday March 08, 2016 @11:41AM (#11246371)
There is a rule for divisibility by 7 - take the last digit, double it, and subtract from the rest of the numbers. If that is divisible by 7, the original number is too. For example, 91 gives 9 - 2 * 1 = 7, so it is divisible by 7
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Re:Is this prime? by EpicDavi on Tuesday March 08, 2016 @12:13PM (#11246640)
Neat trick! Just used it to find out that this post id is divisible by 7.
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