What is the rule to determine if a number is divisible by 3?

The rule for determining if a number is divisible by 3 is based on the sum of its digits. When the digits of a number are added together, if the resulting sum is divisible by 3, then the original number is also divisible by 3. This principle stems from the nature of our base-10 numbering system, where the properties of divisibility can be simplified through the sum of digits.

For example, consider the number 123. If you add the digits (1 + 2 + 3), the sum is 6, which is divisible by 3. Thus, 123 itself is also divisible by 3. This rule is useful for quickly checking divisibility without performing full division, making it a handy tool in mathematics.

Other options provided do not accurately represent the rule for divisibility by 3, focusing instead on different characteristics of numbers.