Given a number, it is a series of digits

`d_1d_2d_3...d_n`

.Add up all the digits

`\sum_{i=1}^nd_i`

.If the resulting number is a multiple of nine (repeat the process if it has more than one digit), or is nine, then the original number is a multiple of nine.

For example, take 81, it is the digits 8 and 1, add the digits,

`8 + 1`

, the result is 9, therefore 81 is a multiple of 9.For another example, take 123456, it is the digits 1, 2, 3, 4, 5, and 6, add the digits,

`1 + 2 + 3 + 4 + 5 + 6`

, the result is 21, which is multiple digits, add the digits, `2 + 1`

, the result is 3, therefore 123456 is not a multiple of 9.
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