A divisibility rule helps you determine if one whole number is divisible by another whole number. Rules for divisibility are quite helpful as they help you to find factors, help you to reduce fractions and to help you find common denominators. Although you may not memorize the divisibility rules, you might just want to keep a copy of them handy.
Examples of divisibility rule are:
for a number to be divisible by 2, the number must end in 0. 2, 4, 6 or 8 or
for a number to be divisible by 3, you simply add the sum of the digits and it must be divisible by 3 ( if 21 is the number, the sum of 2 and 1 is 3 which is divisible by 3 or take 27, 7 + 2 = 9 which is also divisible by 3.