they can help you by finding the two factors of the number given
If the number is even, it is divisible by 2. If the sum of the digits is a multiple of 3, the whole number is divisible by 3.
A number is a multiple of 312 if it's a multiple of 3, 8 and 13 at the same time
Knowing the divisibility rules will help you by being able to recognize if a number has factors (other than one and itself) which are covered by the rules. This will save actually having to start doing divisions.
If the number is even and the sum of its digits is divisible by nine then the number is divisible by 18.
The divisibility rules for a prime number is if it is ONLY divisible by 1, and itself.
You can always check on the divisibility of a number by dividing it into another number. But if you know the divisibility rules, you can get that information easier and faster.
The number 0.
Divisibility rules help you find the factors of a number. Once you've found the factors for two or more numbers, you can find what they have in common. Take 231 and 321. If you know the divisibility rules, you know that they are both divisible by 3, so 3 is a common factor.
divided by what number use your divisibility rules
If the number is even, it is divisible by 2. If the sum of the digits is a multiple of 3, the whole number is divisible by 3.
If a number is divisible by 3 and 5, it is divisible by 15.
Knowing the divisibility rules will help you by being able to recognize if a number has factors (other than one and itself) which are covered by the rules. This will save actually having to start doing divisions.
If the number is even and the sum of its digits is divisible by nine then the number is divisible by 18.
It's not completely necessary to know the factors if the number ends in 1, 3, 7 or 9. You can sum or subtract a certain number of times the last digit by the rest of the number if the number ends in 1, 3, 7 or 9. However I think it's required to factorize the number if it ends in 0, 2, 4, 5, 6 or 8. Here are the divisibility rules of every number from 1 to 50 1: Every number is a multiple of 1 2: The number ends in 0, 2, 4, 6 or 8 3: The sum of the digits is a multiple of 3 4: The last 2 digits are a multiple of 4 The 10s digit is even and the last digit is 0, 4 or 8 The 10s digit is odd and the last digit is 2 or 6 5: The number ends in 0 or 5 6: The number is a multiple of 2 and 3 at the same time 7: The difference between twice the last digit and the rest of the number is a multiple of 7 8: The last 3 digits are a multiple of 8 The 100s digit is even and the last 2 digits are a multiple of 8 The 100s digit is odd and the last 2 digits are 4 times an odd number 9: The sum of the digits is a multiple of 9 10: The number ends in 0 11: The difference between the last digit and the rest of the number is a multiple of 11 12: The number is a multiple of 3 and 4 at the same time 13: The sum of 4 times the last digit and the rest of the number is a multiple of 13 14: The number is a multiple of 2 and 7 at the same time 15: The number is a multiple of 3 and 5 at the same time 16: The last 4 digits are a multiple of 16 The 1,000s digit is even and the last 3 digits are a multiple of 16 The 1,000s digit is odd and the last 3 digits are 8 times an odd number 17: The difference between 5 times the last digit and the rest of the number is a multiple of 17 18: The number is a multiple of 2 and 9 at the same time 19: The sum of twice the last digit and the rest of the number is a multiple of 19 20: The number ends in 00, 20, 40, 60 or 80 21: The difference between twice the last digit and the rest of the number is a multiple of 21 22: The number is a multiple of 2 and 11 at the same time 23: The sum of 7 times the last digit and the rest of the number is a multiple of 23 24: The number is a multiple of 3 and 8 at the same time 25: The number ends in 00, 25, 50 or 75 26: The number is a multiple of 2 and 13 at the same time 27: The difference between 8 times the last digit and the rest of the number is a multiple of 27 28: The number is a multiple of 4 and 7 at the same time 29: The sum of thrice the last digit and the rest of the number is a multiple of 29 30: The number is a multiple of 3 and 10 at the same time 31: The difference between thrice the last digit and the rest of the number is a multiple of 31 32: The last 5 digits are a multiple of 32 The 10,000s digit is even and the last 4 digits are a multiple of 32 The 10,000s digit is odd and the last 4 digits are 16 times an odd number 33: The sum of 10 times the last digit and the rest of the number is a multiple of 33 34: The number is a multiple of 2 and 17 at the same time 35: The number is a multiple of 5 and 7 at the same time 36: The number is a multiple of 4 and 9 at the same time 37: The difference between 11 times the last digit and the rest of the number is a multiple of 37 38: The number is a multiple of 2 and 19 at the same time 39: The sum of 4 times the last digit and the rest of the number is a multiple of 39 40: The last 3 digits are a multiple of 40 The 100s digit is even and the last 2 digits are 00, 40 or 80 The 100s digit is odd and the last 2 digits are 20 or 60 41: The difference between 4 times the last digit and the rest of the number is a multiple of 41 42: The number is a multiple of 2 and 21 at the same time 43: The sum of 13 times the last digit and the rest of the number is a multiple of 43 44: The number is a multiple of 4 and 11 at the same time 45: The number is a multiple of 5 and 9 at the same time 46: The number is a multiple of 2 and 23 at the same time 47: The difference between 14 times the last digit and the rest of the number is a multiple of 47 48: The number is a multiple of 3 and 16 at the same time 49: The sum of 5 times the last digit and the rest of the number is a multiple of 49 50: The number ends in 00 or 50
If a number is divisible by 3, and also by 4, then it is divisible by 12 - so you might use the divisibility rules for those two numbers. Although it might be simpler just to perform the division.
the number is even.