Mathnasium #MathTricks: Divisibility (General Rule for 11s)

To test if a number is divisible by 11, follow the general rule: the difference of the sum of alternating digits can be divided by 11.

To determine if the number 2,728 is divisible by 11, find the difference of the sum of alternating digits. The first sum of the alternating digits is four. The second sum of the alternating digits is fifteen. The difference between the two sums is eleven. Eleven is divisible by eleven, so

2,728 is divisible by 11.

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