Hendecadivisibility
To discover whether a number is divisible by 11, add the digits that appear in odd positions (first, third, and so on), and separately add the digits in even positions. If the difference between these two sums is 0 or a multiple of 11, the original number is divisible by 11. Otherwise it’s not.
For example:
11 × 198249381729 = 2180743199019
Sum of digits in odd positions = 2 + 8 + 7 + 3 + 9 + 0 + 9 = 38
Sum of digits in even positions = 1 + 0 + 4 + 1 + 9 + 1 = 16
38 – 16 = 22
22 is a multiple of 11, so 2180743199019 is as well.
