Question

how 9 is known to be a magic number?​

Answer 1

Nine is a Motzkin number. ... In base 10 a positive number is divisible by 9 if and only if its digital root is 9. That is, if any natural number is multiplied by 9, and the digits of the answer are repeatedly added until it is just one digit, the sum will be nine: 2 × 9 = 18 (1 + 8 = 9)