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(Sum of digits divisible by 9)
A number is divisible by 9 if the sum of its digits is divisible by 9. Same trick as for 3, stricter requirement.
Example: Is 12348 divisible by 9? Sum: 1 + 2 + 3 + 4 + 8 = 18. Is 18 divisible by 9? Yes (18 = 9 × 2). So 12348 is divisible by 9.
Notice: if a number is divisible by 9, it is also divisible by 3. But not the reverse.
