Divisibility rules are simple guidelines that help determine whether one number can be divided by another without leaving a remainder. For example, a number is divisible by 2 if its last digit is even, and it is divisible by 5 if its last digit is either 0 or 5. These rules make it easier to work with numbers, especially when performing calculations or simplifying fractions. Other common divisibility rules include 3 and 9, where a number is divisible if the sum of its digits is divisible by 3 or 9, respectively. Additionally, a number is divisible by 4 if the last two digits form a number that is divisible by 4. These rules are useful for quickly checking divisibility without performing full division.