Cut the number into 2 parts: the last digit and everything else before that. You now have 2 numbers. Take the number that was the last digit and multiply it by 2. Now subtract that from the other number. If this new number is also divisible by 7, then the original number was divisible by 7.
Is 854 divisible by 7?
Number A: 85
Number B: 4
Number A - 2 * Number B = 85 - 2 * 4 = 77
77 is most certainly divisible by 7, therefore 854 is also divisible by 7
Is 854 divisible by 7?
Number A: 85
Number B: 4
Number A - 2 * Number B = 85 - 2 * 4 = 77
77 is most certainly divisible by 7, therefore 854 is also divisible by 7
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