If our teacher wants us to list down the factors of a big number, we have the difficulty of dtermining them. But with the divisibility rules, knowing the factors of big numbers is an easy task.
by 2
- All even numbers are divisible by 2. E.g., all numbers ending in 0,2,4,6 or 8.
- Add up all the digits in the number.
- Find out what the sum is. If the sum is divisible by 3, so is the number
- For example: 12123 (1+2+1+2+3=9) 9 is divisible by 3, therefore 12123 is too!
by 4
- Are the last two digits in your number divisible by 4?
- If so, the number is too!
- For example: 358912 ends in 12 which is divisible by 4, thus so is 358912.
- Numbers ending in a 5 or a 0 are always divisible by 5.
- If the Number is divisible by 2 and 3 it is divisible by 6 also.
- Take the last digit in a number.
- Double and subtract the last digit in your number from the rest of the digits.
- Repeat the process for larger numbers.
- Example: 357 (Double the 7 to get 14. Subtract 14 from 35 to get 21 which is divisible by 7 and we can now say that 357 is divisible by 7.
by 8
- This one's not as easy, if the last 3 digits are divisible by 8, so is the entire number.
- Example: 6008 - The last 3 digits are divisible by 8, therefore, so is 6008.
- Almost the same rule and dividing by 3. Add up all the digits in the number.
- Find out what the sum is. If the sum is divisible by 9, so is the number.
- For example: 43785 (4+3+7+8+5=27) 27 is divisible by 9, therefore 43785 is too!
- If the number ends in a 0, it is divisible by 10.
