ok
you choose a number with n digits called X
(e.g. abc, which means 100a+10b+c)
then you subtract the sum of the digits called Y from it
(e.g. a+b+c)
the result is another number Z
(e.g. 100a+10b+c-a-b-c=99a+9b=9*(11a+b))
=>the number Z is always divisible by 9
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