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Problem
Checking divisibility by 9 for large numbers requires long division.
Solution
Sum all digits. If the sum is divisible by 9, the number is divisible by 9.
Benefit
Quick mental check for divisibility by 9 without calculation.
Example
Is 7,362 divisible by 9?
7 + 3 + 6 + 2 = 18
18 ÷ 9 = 2 ✓
Yes, 7362 is divisible by 9
(In fact: 7362 ÷ 9 = 818)