How to use it
- For 2, inspect the last digit; for 3 or 9, sum digits; for 5, inspect the last digit.
- Apply the relevant test.
- Confirm with division when needed.
See it in action
378 has digit sum 18, so it is divisible by 9; 378÷9=42.
How to verify it
Multiply 42×9 to verify 378.
Common mistake
Each divisor has its own rule; do not generalize one test to all numbers.