What rule determines if a number is divisible by 3?

A number is considered divisible by 3 if the sum of its digits is a multiple of 3. This rule provides a straightforward method to check for divisibility without performing full division, making it especially useful for larger numbers. For example, to determine if 123 is divisible by 3, one would add the digits (1 + 2 + 3 = 6) and check if 6 is a multiple of 3, which it is. Therefore, 123 is also divisible by 3.

This method is applicable regardless of the number size and is a commonly taught arithmetic shortcut. Understanding why this rule holds true involves concepts from number theory related to the base-10 number system and how numbers can be expressed as sums of their digit contributions to place values.

The other options do not accurately represent rules for divisibility by 3. A number ending in 0 indicates divisibility by 10, whether it is odd does not relate to divisibility by 3, and a prime number is defined by its divisibility by itself and 1, not specifically by 3.