Question

Why is 9 considered to be equal to 0 in the digital sum method?

Answer 1

Answer:

The sum of the digits of any multiple of nine (other than zero) equals another multiple of 9, and repeated summing of digits eventually leads to 9. The reason for this has to do with how the decimal system works. When one place value gets larger than 9, it goes back to 0 and you add 1 to the next place value.

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