The book is now available! 7
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Divisibility test for 7 (or 13): Combine the digits in order into groups of 3 (starting from the right) by alternating them with positive and negative signs. If the result is divisible by 7 (or 13 respectively), then so is the original number. For example, 7 divides 62540982 because 7 divides +(62)-(540)+(982).

 

  Submitted: 2008-09-05 11:19:07;   Last Modified: 2008-09-05 11:22:02.



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