What is the divisibility rule for 9?

Answer 1

If you add up the digits, and the sum is divisible by 9 or a multiple of 9.

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For really big numbers you may need to find the digital root (sum of digits) of the digital root.
The divisibility rule for 9 is that the sum of the digits of a number must add up to 9 or a multiple of 9.

For example, is 81 divisible by 9? Lets find out! 8+1=9. Then it is divisible by 9.

Answer 2

If the number is divisible by 3 then its divisible by 9. If you forgot what the divisble rule for three this is how, add up all the numbers then whatever the sum is and if the sum can go into three then its divisible by 3 and 9.