Why is modulo 9 the Digital root of all numbers?

Question

heres what i mean:

132189 –> 1 + 3 + 2 + 1 + 8 + 9 = 24 –> 2 + 4 = 6
132189 % 9 = 6

https://en.wikipedia.org/wiki/Digital_root

In: 7

Answer 1

It is not. read the wiki: In base 10, this is equivalent to taking the remainder upon division by 9 (except when the digital root is 9, where the remainder upon division by 9 will be 0).