Double the last digit. Subtract it from the rest. If the result is divisible by 7, the original number is divisible by 7.
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Double the last digit and subtract it from the others.
Example: 357
7 x 2 = 14
35 -14 = 21
21 is a multiple of 7.
357 is a multiple of 7.
49/7 = 7/7 = 1 7 x 7 = 49
1, 5, 7 and 35
The Rule - 2009 Caroline Pruitt Bear Hunt 2-7 was released on: USA: 9 August 2010
Starting with the numeral 7, triple it. Double the answer. Triple that answer. Double that answer . . . . .
The same as the greatest common factor for 77 and 7, where 7 is the remainder of the division of 161 by 77.