Divisibility Rules
- Divisible by 1: Whole number
- Divisible by 2: Even
- Divisible by 3: Sum of digits must be divisible by 3 (3, 6, 9, 12, etc)
- Divisible by 4: Divisible by 2 twice
- Divisible by 5: Ends with 5 or 0
- Divisible by 6: Divisible by 2 & 3
- Divisible by 7: Take off the last digit, double that number, then take the doubles number and subtract the original last digit
- Divisible by 8: Divisible by 2 three times
- Divisible by 9: Sum of digits are divisible by 9 (9, 18, 27, 36, etc)
- Divisible by 10: Ends with 0
Note that the rules for 3, 7, and 9 can be repeated infinitely, if you don't know if the result is divisible.