The easiest rule is for 1: every number is divisible by 1! The only rule to check if 1 divides evenly into any number, is just to check if you actually have a number!
Numbers that are divisible by 2 are called even numbers. To see if a number is even, look at the last digit, ie the one's digit. If the last digit is 0, 2, 4, 6, or 8, then you've got an even number.
To tell if a number is divisible by 3, add up all the digits in the number. If the sum is divisible by 3, then so is the original number. For example, take the number 2339088. The sum of the digits is 33. Since 33 is divisible by 3, so is 2339088.
To check if 4 will divide evenly into a number, look at the last two digits. If the last two digits form a number that is divisible by 4, then the whole number is also divisible by 4. For example, take the number 2339088. The last two digits are 88, which is a number divisible by 4. Thus 4 will also go into 2339088.
The rule for multiples of 5 is to check the last digit. If it's 5 or 0, then the number is a multiple of 5.
How can you tell if 6 will go into a number evenly? Just apply the rules for 2 and 3. If the number is both even and divisible by 3, then the number is also divisible by 6.
Unfortunately, there is no simple trick for telling if a number is divisible by 7. This has to do with the base of the number system we count with, which in our case is base 10. In number systems with other bases, such as base 6, 8, or 14, there are rules for telling when a number is divisible by seven, like the rules above.
To tell if a number is divisible by 8, look at the last 3 digits. If the last three digits form a number divisible by 8, then the whole is divisible by 8. For example, 576128 is divisible by 8 because 128 is.
A number is divisible by 9 when all the digits add up to a multiple of 9. For example, take the number 8077122. The sum of its digits is 27. Since 27 is a multiple of 9, so is 8077122.
The rule for 10 is simple, just check if the number ends in 0.
To see if 11 will divide evenly into a number, alternately subtract and add the digits of the number and see if the result is a multiple of 11. For example take the number 7292615. Now compute 7-2+9-2+6-1+5, which gives 22. Since 22 is a multiple of 11, then 7292615 is as well. (Note, if the sum is 0, -11, -22, -33,... etc, the original number is still divisible by 11.)
The last rule in this list is for 12. To see if 12 will go into a number, simply apply the rules for 3 and 4.
Divisibility rules can help you reduce fractions and ratio, and they can also help you determine more quickly if a number is prime or composite.