The divisibility rule for 9 states that a number is divisible by 9 if the sum of its digits is divisible by 9.
For example, consider the number 729. To determine if it is divisible by 9, we first add its digits: 7 + 2 + 9 = 18. Since 18 is divisible by 9, we can conclude that 729 is also divisible by 9.
This rule can be used for any number, no matter how large. Just sum up all the digits, and check if that sum can be divided evenly by 9. If it can, then the original number is divisible by 9!